Vegetarian Breakfast Casserole

Bursting with flavor, this Vegetarian Breakfast Casserole is easy to make for breakfast that’s also great for meal prepping. Packed with veggies and protein, this casserole is like a crustless quiche. Enjoy the combination of zucchini, onion, tomatoes, and basil with 2 kinds of optional cheeses to add in as well. We love meal prepping this for quick grab-and-go breakfasts. But it’s simply gorgeous for slow weekend breakfasts and brunches too.

Loaded with veggie goodness, this breakfast casserole is like an easy crustless quiche that’s naturally gluten-free. It’s a favorite in our house for entertaining because it’s colorfully appealing. But it also makes a great quick breakfast straight out of the fridge.

Ingredients & Substitutions

• Eggs – provide the bulk structure of this breakfast casserole and there isn’t a good substitute for them in this egg-based casserole.
• Butter – I like to use salted butter or ghee in this casserole but olive oil works well too.
• Zucchini – adds so much texture and the perfect amount of moisture. I recommend some caution when subbing in other vegetables here because it’s a bit tricky. But if you do want to use something else like broccoli etc. you’ll need to make sure they’re cooked before adding them to this casserole. There’s not enough baking time for firmer veggies to get cooked, so they need to be pre-cooked if you want to sub in something else. A few veggies that would be easy to sub in would be mushrooms or diced red bell peppers which you could cook briefly with the onions. Or add 2 cups roughly chopped spinach to the pan with the onions just until barely wilted in place of the zucchini.
• Onion – I like to use red onion because it’s visually appealing but you can use any other kind of onion including shallots and green onions. If you use green onions, there’s no need to cook them first.
• Tomatoes – cherry tomatoes work best in this casserole because they tend to hold their shape better than diced larger tomatoes. I used a mix of Sungold and red cherry tomatoes, but any cherry tomato works.
• Basil – is our favorite fresh herb and it works really well with these veggie flavors. You could also use dill or parsley here.
• Salt & pepper – you can adjust this to your taste if desired. But be aware it’s hard to season an egg casserole after it’s baked. For the best results, the salt and pepper need to be added in so the whole casserole is properly seasoned. Otherwise, you may find it to be bland.

Step 1: Start by preheating the oven, then grate the zucchini and set it aside.

Step 2: Slice onion into half-moon slices.

To do this start by slicing off both ends of the onion and peeling away the outer layer. Then slice the onion in half from the top to the bottom. Lay 1/2 of the onion flat side down on the cutting board and cut across the rounded top to create half-moon slices.

Step 3: Then briefly cook it in butter in a large skillet (9″) over medium heat. This is essential to take the raw onion edge off because the baking time isn’t quite enough.

You can eliminate this step if you use thinly sliced green onions which can be added straight to the egg mixture.

Also if you have a very well-seasoned cast iron skillet, you can bake the egg mixture right in the same pan you use to cook the onions. (Note: I don’t recommend cooking egg-based casseroles in enameled cast iron skillets because they stick.)

Step 4: Next whisk the eggs together well with the salt and pepper in a large bowl. Then stir in the veggies and chopped basil.

If desired reserve some of the cherry tomatoes to place by hand on top of the egg bake for presentation.

Step 5: Pour the veggie egg mixture into a well-buttered pie plate or similar-sized baking dish. If you’re using the optional cheese, you can sprinkle it over the top or mix it in.

Top with the cherry tomatoes for presentation if you reserved them. This isn’t necessary by any means, it’s only for appearance!

Step 6: Bake the egg casserole until puffed in the center and done.

Serve hot out of the oven, or at room temperature. Let the casserole cool to just warm if you want to portion it out for meal prep.

Ways to Serve this Vegetarian Breakfast Casserole

• Serve this Veggie Breakfast Casserole recipe on its own with simple accompaniments like fresh juice or seasonal fresh fruit.
• Make it a special occasion breakfast or brunch by adding my Paleo Blueberry Muffins or Almond Flour Raspberry Muffins to the menu. If you have lots of zucchini to use up, pair this casserole with my Healthy Chocolate Zucchini Muffins.
• Add additional protein with your favorite sausages or bacon. Try my Air Fryer Sausage Links or my Turkey Breakfast Sausage. Make it an all-out breakfast with my Best Rosemary Potatoes paired with the sausage.
• Serve this casserole with my Fruit and Yogurt Parfaits for a stunning brunch. You can use fresh fruit with that recipe or make either my Berry Sauce or Strawberry Rhubarb Sauce.

Storing leftovers

• To store – place any leftovers into an airtight container and store them in the refrigerator for 4-5 days.
• Make ahead – this recipe keeps well in the fridge and can be made ahead for meal prep.
• Reheating – egg casseroles are tricky to reheat. The microwave is the easiest. But you can also add a slice to a nonstick skillet along with 1-2 tbsp of water. Cover and simmer on low until the casserole is heated through and the water has evaporated. Once the slice of casserole is almost heated through, I like to add a slice of cheese on top and cook covered just until the cheese melts.
• To freeze – this casserole doesn’t hold up in the freezer well because of the veggies. I would try my Sausage Hash Brown Breakfast Casserole instead which freezes pretty well. Egg casseroles are never quite as good after they’ve been frozen.

Dietary Options & Substitutions

• Dairy-free – use olive oil or dairy-free butter. Don’t use the optional cheese, and I personally don’t recommend vegan cheese in this casserole.
• Paleo – use ghee or olive oil.
• Vegetarian – use olive oil or vegan butter.

More Egg Breakfast Recipe Inspiration

• Sausage Hash Brown Breakfast Bake
• Make Ahead Sausage Egg Muffins
• Meal Prep Egg Muffins 3 Ways
• Red Pepper Feta Frittata
• Veggie Scrambled Eggs