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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I made this with homemade almond milk and it was a little watery. Does it get thicker with store-bought almond milk? And have you tried it without the almond milk at all — just the eggs? Thanks!
Yes you can make this recipe with only eggs and yes it get thicker with store-bought almond milk.
Can you make this recipe the night before and bake it the next day?
Yes you can!
If made in a 9×13 baking dish, would you double all ingredients?
What do you mean by make ahead? Leave uncooked in refrigerator or make ahead, cook and warm up later?
You can leave uncooked in the fridge overnight. You can also cook and warm up later.
Sounds easy & yummy!
It’s really good!
Can I make this recipe two days ahead? Just put everything together and put in fridge until I’m ready to cook it???
yes you can!
Can you make this recipe with 2% cow milk or must you use almond milk?
Can you make this recipe with 2% cow milk or must you use almond milk?
Yes you can 😉
If made in a 9×13 baking dish, would you double all ingredients?
What is good to serve with this wonderful casserole to vegetarians?
With a delicious salad and roasted veggies.
Can you make this recipe with skim milk instead of almond milk?
Yes you can.
Easy and good. like that you can add just about anything you want.
Absolutely!
Served for Christmas Morning- and just made 3 for Easter brunch this year! Easy & delicious! Thank you!!
You’re so welcome! So happy you made it again for Easter. Thanks for stopping by 😉
Can we use regular milk instead of almond milk?
Yes you can 😉
When reheating in the oven, what temp and for how long?
Great question! To reheat it in the oven, warm it at 350°F (180°C) for about 15–20 minutes, or until heated through. If it was refrigerated, you can cover it with foil so it doesn’t dry out. 😊
How many does it serve? I need it to serve 30 people
This serves 8 people!