Heat olive oil in a skillet and sauté the onion, bell pepper, and mushrooms until soft. Stir in the chopped spinach at the very end just until it wilts. This step is essential for a top-notch high-protein vegetarian breakfast because it removes excess moisture that would otherwise make your casserole soggy.
Grease a 9×13-inch baking dish and spread the frozen hash browns in an even layer across the bottom. This creates a sturdy, potato-rich “crust” that holds the eggs and vegetables together, making it easy to slice into perfect squares for meal prep.
In a large bowl, whisk together 10 eggs, milk, salt, pepper, garlic powder, and smoked paprika. Beat the mixture for two minutes until it is frothy. Incorporating air at this stage ensures the casserole emerges from the oven light and fluffy rather than dense.
Sprinkle half of the shredded cheddar directly over the hash browns. This acts as a savory binder between the potatoes and the eggs. Using a sharp cheddar provides a bold flavor that stands out against the mildness of the eggs and fresh vegetables.
Distribute the sautéed vegetables over the cheese layer, then slowly pour the egg custard over everything. Use a fork to ensure the liquid reaches the bottom. Top with the remaining cheese to create a golden, bubbly crust during the bake.
Bake at 375°F for 45 to 55 minutes until the center is set. Let the casserole rest for at least 15 minutes before slicing. This patience is rewarded with clean, professional-looking portions that maintain their shape and protein-packed structure.
