Preheat oven to 375°F and grease a 7×11-inch or 8×11-inch baking dish with nonstick spray or butter.

Crumble the breakfast sausage into a skillet and cook over medium heat until browned and fragrant.

Use a slotted spoon to transfer browned sausage to a plate, leaving excess fat behind.

In a large bowl, whisk together eggs, milk, salt, pepper, garlic powder, and smoked paprika until smooth and pale yellow.

Scatter cooked sausage evenly in the bottom of the prepared baking dish.

Sprinkle diced bell peppers, chopped spinach, and onions over the sausage if using for added color and nutrients.

Evenly layer shredded cheese on top of the sausage and veggies.

Distribute bread cubes across the dish to soak up the custard and add texture.

Pour the seasoned egg mixture gently over everything, ensuring the liquid seeps into all corners.

Place casserole in the preheated oven and bake until the center is set and the top is lightly golden, about 30–40 minutes.

Let the casserole rest 5 minutes before slicing so each piece holds together beautifully.

Serve warm and enjoy the rich blend of savory sausage, fluffy eggs, and melty cheese.