Vegetarian Breakfast Casserole With Bell Peppers & Onions – Easy, Hearty, and Make-Ahead Friendly

This vegetarian breakfast casserole brings together tender eggs, sautéed bell peppers, and sweet onions for a cozy, crowd-pleasing dish. It’s the kind of breakfast that feels special but doesn’t require much effort. You can prep it the night before, pop it in the oven in the morning, and let it bake while you make coffee.

It’s perfect for weekends, brunch with friends, or a simple weekday meal you can reheat all week. If you like a savory, colorful start to your day, this one checks all the boxes.

What Makes This Recipe So Good

• Simple prep, big payoff: A few chopped veggies, whisked eggs, and cheese come together into a hearty bake with minimal fuss.
• Balanced and satisfying: Protein from eggs, fiber from vegetables, and optional whole-grain bread or potatoes keep you full for hours.
• Flexible ingredients: Use any color bell peppers, add spinach or mushrooms, or swap cheeses—this casserole is very forgiving.
• Make-ahead friendly: Assemble the night before and bake in the morning, or bake ahead and reheat slices as needed.
• Great for feeding a crowd: Serves 6–8, and scales up easily for larger gatherings.

What You’ll Need

• 8 large eggs
• 1 cup milk (dairy or unsweetened plant-based; whole milk gives a richer texture)
• 1 red bell pepper, diced
• 1 green bell pepper, diced (use yellow or orange if preferred)
• 1 medium yellow onion, diced
• 2 cups baby spinach, roughly chopped (optional but recommended)
• 1 1/2 cups shredded cheese (cheddar, pepper jack, or mozzarella)
• 2 cups cubed day-old bread or cooked diced potatoes (choose one; bread for a softer, strata-like bake, potatoes for a sturdier casserole)
• 2 tablespoons olive oil or butter
• 1 teaspoon garlic powder (or 2 cloves garlic, minced)
• 1 teaspoon smoked paprika (adds warmth and depth)
• 1/2 teaspoon dried oregano or Italian seasoning
• Salt and black pepper, to taste
• Fresh herbs for serving (chives, parsley, or cilantro), optional
• Hot sauce or red pepper flakes, optional for heat

Step-by-Step Instructions

1. Preheat and prep: Heat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with oil or butter.
2. Sauté the vegetables: Warm 2 tablespoons olive oil in a large skillet over medium heat.

   Add onion and bell peppers with a pinch of salt. Cook 6–8 minutes, stirring occasionally, until softened and slightly caramelized. Stir in garlic powder (or minced garlic), smoked paprika, and oregano.

   Cook 1 minute more.

3. Wilt the greens: Add chopped spinach and cook 1–2 minutes until just wilted. Remove the skillet from heat and let the vegetables cool slightly.
4. Layer the base: Spread the cubed bread or cooked potatoes evenly in the greased baking dish. Scatter the sautéed vegetables over the top.
5. Whisk the eggs: In a large bowl, whisk eggs, milk, 1 teaspoon salt, and 1/2 teaspoon black pepper until smooth and well combined.
6. Add the cheese: Stir 1 cup of the shredded cheese into the egg mixture.

   Reserve the remaining 1/2 cup for topping.

7. Combine and pour: Pour the egg mixture over the vegetables and bread/potatoes, ensuring everything is evenly coated. Gently press down any pieces sticking out so they’re submerged.
8. Top and bake: Sprinkle the remaining cheese on top. Bake uncovered for 30–40 minutes, until the center is set and the top is lightly golden.

   A knife inserted in the middle should come out mostly clean, with no runny egg.

9. Rest and serve: Let the casserole rest for 10 minutes. Garnish with fresh herbs, add hot sauce if you like, then slice and serve warm.

How to Store

• Refrigerate: Cool completely, then cover the dish or transfer slices to an airtight container. Store for up to 4 days.
• Reheat: Microwave individual slices for 60–90 seconds, or warm in a 325°F (165°C) oven for 10–15 minutes until heated through.
• Freeze: Wrap individual portions tightly and freeze for up to 2 months.

  Thaw overnight in the fridge and reheat as above.

Benefits of This Recipe

• Vegetable-forward: Bell peppers and onions add fiber, color, and natural sweetness without extra sugar.
• Protein-packed: Eggs and cheese keep you energized and satisfied, making it a solid breakfast or brunch option.
• Customizable for dietary needs: Easily make it gluten-free by using potatoes instead of bread and choosing certified gluten-free ingredients.
• Budget-friendly: Uses common, affordable ingredients that stretch to feed a family.
• Meal prep win: Bake once, enjoy several times throughout the week with quick reheats.

Pitfalls to Watch Out For

• Watery casserole: If veggies release too much liquid, the texture suffers. Sauté until moisture cooks off before assembling.
• Underseasoning: Eggs need salt. Taste your veggies as you cook them and season the egg mixture well.
• Rubbery texture: Overbaking dries out the eggs.

  Pull it from the oven when the center is just set.

• Soggy base: If using bread, choose day-old or lightly toast fresh bread so it holds up.
• Uneven cooking: Overcrowding or very thick layers can lead to a jiggly center. Use the right pan size or bake a bit longer, checking often.

Recipe Variations

• Southwest style: Add black beans, corn, and pepper jack. Finish with cilantro and a squeeze of lime.
• Mediterranean: Stir in chopped sun-dried tomatoes, olives, and feta.

  Swap oregano for za’atar or Italian seasoning.

• Broccoli cheddar: Use lightly steamed broccoli florets and sharp cheddar for a classic combo.
• Mushroom and thyme: Sauté sliced mushrooms until browned and add fresh thyme for an earthy flavor.
• Spicy kick: Mix in diced jalapeños or red pepper flakes and top with hot sauce.
• Dairy-free: Use unsweetened almond or oat milk and a dairy-free cheese you enjoy, or skip cheese and add extra veggies.
• No-bread version: Use roasted potatoes or sweet potatoes as the base for a gluten-free, sturdier casserole.

FAQ

Can I make this casserole the night before?

Yes. Assemble it fully, cover, and refrigerate overnight. In the morning, let it sit on the counter for 15–20 minutes while the oven preheats, then bake.

You may need to add 5–10 extra minutes to the cook time since it’s starting cold.

How do I know when the casserole is done?

The center should be set, not liquidy, and the top lightly golden. A knife or toothpick inserted in the middle should come out mostly clean. If it looks wet, bake a few minutes longer and check again.

Can I use egg whites only?

You can, but the texture will be lighter and less rich.

Use about 2 cups of liquid egg whites to replace 8 whole eggs, and consider adding an extra 1–2 teaspoons of olive oil for moisture.

What’s the best cheese for this recipe?

Cheddar melts nicely and adds flavor, while mozzarella gives a gooey texture. Pepper jack adds a mild heat, and feta offers a tangy, salty bite. Use what you like or mix two for balance.

Do I need to peel the bell peppers?

No.

The skins soften in the oven, and peeling isn’t necessary. Just remove the seeds and ribs, then dice evenly so they cook at the same rate.

Can I add more vegetables?

Absolutely. Just sauté watery vegetables like mushrooms or zucchini until the liquid cooks off.

Keep the overall volume of add-ins similar so the eggs still set properly.

How can I make it higher in protein?

Add more eggs (up to 10), include cottage cheese in the egg mixture, or use a higher-protein cheese. You can also serve it with Greek yogurt on the side.

Wrapping Up

This Vegetarian Breakfast Casserole with Bell Peppers and Onions is dependable, colorful, and easy to adapt. It works for lazy mornings, busy weekdays, and everything in between.

With a few simple ingredients and a short bake time, you get a warm, satisfying meal that keeps well and tastes great. Make it once, and it will quickly become a go-to in your breakfast rotation.