Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter.

In a skillet over medium heat, cook the breakfast sausage until fully browned. Break it into small crumbles as it cooks. Drain excess grease and set aside.

If using onions or bell peppers, sauté them in the same skillet until soft.

In a large mixing bowl, whisk together eggs, milk, garlic powder, salt, and pepper until well combined.

Spread the cooked sausage evenly across the bottom of the greased baking dish. Add sautéed vegetables if using, then sprinkle shredded cheese on top. If using bread cubes, distribute them evenly now.

Pour the egg mixture evenly over the layers in the baking dish. Gently shake the dish to help the liquid settle.

Bake in the preheated oven for 30-35 minutes, or until the eggs are set and the top is golden brown. To check for doneness, insert a knife into the center—if it comes out clean, the casserole is ready.

Let the casserole rest for 5 minutes before slicing and serving. Enjoy!