This Make-Ahead Breakfast Casserole is the perfect solution for busy mornings when you still want something wholesome and delicious. Packed with colorful veggies, fluffy eggs, and melty cheese, it bakes up golden and satisfying every time. The best part? You can prep it the night before, pop it in the oven the next morning, and breakfast is ready with zero stress. It’s great for weekend brunch, Christmas morning, or easy meal-prep for the week!
Olivia’s Personal Note
I love making breakfast casseroles, and some of my favorites like my Breakfast Casserole with Potato and Sausage, Vegetable Frittata, and Spinach Mushroom Frittata are always on repeat in my kitchen.
They’re easy, versatile, and made with fresh, seasonal ingredients, which is exactly why I adore them.
I honestly think this would make a wonderful Christmas morning breakfast for two big reasons: first, it’s the perfect way to use up leftover veggies from your holiday meals; and second, it frees up your whole morning so you can open gifts, enjoy time with your family, and soak in all the Christmas magic.
This Breakfast Vegetarian Casserole also makes an amazing overnight casserole. Just prep everything the night before. Chop and sauté the veggies, whisk the eggs, and assemble. Pour the egg mixture over the veggies, add the cheese, cover with foil, and refrigerate overnight. Then in the morning, all you have to do is bake and enjoy. Perfect for slow, cozy holiday mornings.
Ingredients to Make Ahead Breakfast Casserole
All ingredients and their amounts are listed in the recipe card at the bottom of this post.
Vegetables: I’ve tested this casserole with all kinds of veggie combinations, and this mix gives the best balance of flavor, color, and texture. The red onion adds sweetness, broccoli brings crunch, mushrooms add savory depth, cherry tomatoes give juiciness, and spinach adds freshness. I’ve found that chopping everything into bite-sized pieces helps the veggies cook evenly and prevents the casserole from becoming watery.
Eggs: The eggs are what hold the casserole together and give it that light, fluffy texture. I also like making this casserole with whole eggs instead of adding extra egg whites. I’ve found that full eggs give the casserole a richer flavor and a creamier, fluffier texture. They also help everything bind together beautifully.
Non-dairy milk (or any milk of choice): I prefer using unsweetened almond milk to keep the casserole a bit lighter, but I’ve tested it with whole milk, oat milk, and even half-and-half. Dairy milk adds richness, while almond milk keeps the texture soft and neutral. It’s a very flexible recipe depending on your dietary needs.
Cheese: I usually use a mix of cheddar and mozzarella. After testing a few blends, I’ve found cheddar gives great sharpness and flavor, while mozzarella melts smoothly and adds creaminess. You can swap in Swiss, Monterey Jack, or even pepper jack if you want a little spice.
Instructions to Make This Vegetarian Breakfast Casserole:
Prepare the equipment
- Preheat oven to 325°F.
- Spray a 9×9-inch casserole dish or baking pan with a good coating of cooking spray, and set aside.
Cook the vegetables
- In a large skillet over medium heat, add the olive oil and then add the onions, and cook until they are soft.
- Then, add the rest of the veggies, stir well, and sauté until they are tender.
Transfer everything to the baking dish and bake
- Add the sauteed veggie mixture to the baking dish.
- Whisk together the eggs, almond milk, garlic powder, mustard powder, salt, and black pepper in a large bowl.
- Pour the egg mixture over the veggie mixture in the baking dish. Sprinkle with cheddar cheese (or any other you have on hand) and cover with foil. Bake for 40 minutes.
- Remove the foil, and cook an additional 10 minutes, or until the egg mixture is cooked through.
- Let your egg casserole cool for 10 minutes before serving. Top with chopped parsley, green onions or basil.
Olivia’s Recipe Tips
Let your egg mixture rest for 5 minutes before pouring it over the veggies. This simple step helps the proteins relax and creates a smoother, creamier texture.
Season the veggies—not the eggs. Salt pulls moisture out of vegetables, but when added directly to eggs, it can thin the custard.
For a firmer casserole, add one extra egg. Great for slicing and packing into lunchboxes or storing for meal prep.
For a softer, custard-like casserole, add extra milk. An extra ¼ cup milk gives a more luxurious texture.
Add frozen veggies (with a trick!). You can use frozen. Just thaw completely, squeeze out the water, and pat dry.
Make Ahead Breakfast Casserole
Video
Ingredients
- 2 tablespoons extra virgin olive oil
- 1 cup red onions chopped (skip red onions if you're folling a keto diet)
- 2 cups broccoli chopped
- 3 cups mushrooms sliced or chopped
- ½ cup cherry tomatoes
- 2 cups fresh spinach
- 6 eggs
- ½ cup almond milk
- ½ teaspoon garlic powder
- ¼ teaspoon mustard powder optional
- Salt and pepper to taste
- 1 cup cheddar and mozzarella cheese mix
- Parsley for garnishing (optional)
Instructions
- Preheat oven to 325°F. Spray a 9×9-inch casserole dish with a good coating of cooking spray, and set aside.
- In a medium skillet, add the olive oil over medium heat. Add the onions, and cook until they are soft. Then, add the rest of the veggies, and sauté until they are tender. Add this veggie mixture to the baking dish. Set aside.
- While the veggies are cooking, whisk together the eggs, almond milk, garlic powder, mustard powder, salt, and pepper in a bowl.
- Pour the egg mixture over the veggie mixture in the baking dish. Next, add the cheese.
- Cover with foil, and bake 40 minutes. Remove the foil, and cook an additional 10 minutes, or until the egg mixture is cooked through. Let it cool for 5 minutes before serving.
Notes
- To store: Leftovers can be kept in a container in the fridge for up to 5 days.
- To reheat: Reheat the casserole in the microwave or in the oven.
- To freeze: Wrap up the casserole tightly and freeze for up to 3 months. You can reheat it once fully thawed.
Nutrition
Nutrition information is automatically calculated, so should only be used as an approximation.
Did You Make This?
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Can you double this recipe? Would you exactly double the ingredients and use a 9×13 pan?
Probably you can.
I made two of these today. One for my family and one for another family. I didn’t us mushrooms, as we HATE mushrooms. But I added yellow squash and red pepper flaks for some spice. It turned out perfect! Thanks for posting this as it gave me something to build upon. Next time I think I will add some green onion to enhance the taste.
You’re very welcome, Donia. Happy to help 😉
If you cook it and leave it in the fridge overnight, what temp and how much time does it need in the oven to heat up on the morning?
Pre-heat the oven at 350F in the oven for 10-20 minutes. It will depend on the power of your oven.
Que gostiso
Muito bom mesmo.
Can you use regular milk? Thx 🙂
Yes you can.
Is the almond milk plain? Or could one get away with unsweetened vanilla? And could one use edge whites only?
Yes the almond milk is plain (no vanila flavour). And yes you can use egg whites to make this recipe.
I am on a strict diet, just trying to lose weight. So, I made a few alterations and it was really good. I used green onions instead of red onions. Along with spinach, broccoli and mushrooms, I added cilantro. Left out the milk and added about 1/4 c. water. I didn’t add cheese to the top but could have put it on 1/2 of it for others in my family. It was really, really good. Thin, like a quiche, maybe. But my husband and I ate it up and I put extra in the fridge for later.
That’s awesome, Claire. Happy to hear that. Thanks for stopping by and leave your feedback 😉
For the ‘make ahead’ part – would you recommend cooking the night before and reheating the casserole, or assembling it and then sticking it in the fridge overnight, baking the next morning? Thanks!
Both ways work great. If you have a very busy morning is better you cook it the night before. If not you can only assemble it the night before and bake next day. It’ll be fresh, warm and delicious 😉
Can this be made ahead and frozen and/or made ahead, baked, and frozen? My son made it this morning. It is FABULOUS. I am sharing it with as many people as possible. A real WINNER. Thank you. PS: Gave it five-stars only because that was the highest rating. It certainly deserves at least TEN.
Wow so sweet. Thank you so this incredible review. It made my day.
Yes you can make ahead, place in a casserole, wrap it tightly with plastic wrap and put in the freezer. Remove from the freezer and thaw in the refrigerator overnight. Bring to room temperature 30 minutes before cooking. Bake, uncovered until top is light golden and the center is cooked through.
GREAT RECIPE! Didn’t have broccoli, used zucchini instead. Added garlic powder, fresh parsley and some cheese to egg mixture. It tasted just like Quiche…..so yummy!😍
Right? It’s delicious 😉
A small amount of Romano cheese will add a lot to the flavor. It somehow enhances the flavor of the other cheeses.
Totally agree. You can add cheese to this recipe, but I made it without cheese because I’m trying to avoid dairy.
I just made it this morning- so easy and delicious!! I nuked frozen chopped spinach, drained it and added it to my cooked onions and mushrooms. I was out of almond or regular milk so I added 1/2 cup cottage cheese and 1/4 cup liquid egg whites. (Dairy is fine for me). I wasn’t sure about the mustard powder so I left it out and added some poultry seasoning. Next time I will try mustard- it might be great.
I am going to whip this up on Sunday nights so I’ll have a quick breakfast during the week. So yummy! Thank you
Awesome. Happy to hear that 😉
This sounds delicious. I plan on making this for my daughter (she just had her first child) Is it better to cook this before freezing?
Sorry Tomiko, but I’m not sure if this recipes freezes well. After you cook, it’s good in the fridge for 4 days.
Can this be made ahead and frozen uncooked?
I believe so but I’ve never frozen this recipe uncooked before. I can’t really confirm 100% that it will work.
Did you dice all the vegetables? How did you cut the broccoli? Do i have to dice the onion and mushroom?
Also how did you measure 0.75 egg? Did you only use the egg yolk or egg white?
I used 6 whole eggs.
Yes you have to dice/chop all the veggies.
Egg-cellent recipe and delicious!I used about 7 different veggies, including tomato and zucchini. I used 8 eggs and topped with sharp cheddar, feta and grated Parmesan cheeses. Due to the extra veggies I cooked it an extra 10 minutes. The result was so amazing! Great for breakfast or any meal with a dab of salsa. I’m subscribing for more of your great recipes.
Thank you for helping me cook healthy!
Thank you for your comment and for making this recipe. It’s really good. Also, thanks for subscribing to my newsletter and for all the support. I appreciate it a lot 😉
Assembled last pm, stuck in fridge, and just woke up and popped in the oven. Looking forward to eating a delicious, quick n easy, healthy thought-out breakfast for Christmas morning! Thanks so much for posting!
you’re very welcome. Hope you liked it!
This recipe is so good. I am trying to squeeze more fruits and veg into my diet and this is perfect to make ahead (I made a couple of casseroles at the same time and froze the individual pieces then vacuum sealed them). So perfect to heat up quickly in the morning! I love that you can make it anyway you want– I made it with mushrooms, leeks, asparagus, spinach, and sun dried tomato with gruyere cheese on top. So yummy!
Mmm.. gruyere cheese works very well with this recipe. Happy you liked it.
This recipe is a weekly mainstay for me. I change up the veggies and the cheese. It is so yummy. My family loves it and looks forward to eating it (never any leftovers), even my two-year old twin grandchildren love it! Thanks for all the great recipes.
Your comment made my day. Thank you so much for stopping by and for leaving your feedback. I appreciate it!
How to make without oven
You can cook it in a skillet on low temperature and close the skillet with a lid. Be careful to not burn the frittata!
Can you use crescent rolls then add egg mixture??
I believe you can, but I’m not completely sure just because I’ve never made this recipe with crescent rolls. But let me know if thy try it. I would like to know how it turned out. Thanks