Preheat the Oven: Start by preheating your oven to 350°F (175°C). This ensures that your casserole cooks evenly and comes out perfectly golden.
Cook the Sausage: In a large skillet over medium heat, add the breakfast sausage. Cook, breaking it apart with a spatula, until it’s browned and fully cooked (about 5-7 minutes). Drain any excess fat and set aside.
Sauté the Vegetables: In the same skillet, add a splash of oil (if needed) and toss in the diced onion and bell peppers. Sauté for about 3-4 minutes until they’re soft and fragrant. If you’re using spinach, add it in the last minute of cooking to wilt it slightly.
Whisk the Eggs: In a large mixing bowl, crack the eggs and whisk them together. Add the cooked sausage and sautéed vegetables, along with the salt, pepper, and garlic powder. Mix until everything is well combined.
Layer the Tortillas: Grab a baking dish (about 9x13 inches). Place two flour tortillas in the bottom, overlapping them slightly to cover the entire base. This will be the foundation of your casserole.
Add the Egg Mixture: Pour half of the egg and sausage mixture over the tortillas, spreading it evenly. Sprinkle one cup of shredded cheese on top.
Repeat the Layers: Add two more tortillas on top of the cheese, followed by the remaining egg mixture. Top with the remaining cheese to create a gooey, cheesy finish.
Bake: Cover the baking dish with aluminum foil to prevent browning. Bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Cool and Serve: Once done, let the casserole cool for about 5 minutes before slicing. This helps it set and makes serving easier.