Preheat your oven to 375°F (190°C). Lightly grease a 7×11-inch or 8×11-inch baking dish with butter or nonstick spray.
Brown the sausage in a skillet over medium heat, breaking it into crumbles. Drain excess grease and set aside.
In a large bowl, whisk together the eggs, milk, salt, black pepper, garlic powder, and smoked paprika until well combined.
Layer the cooked sausage evenly in the bottom of the prepared baking dish. Add diced bell peppers, spinach, and onions if using.
Sprinkle the shredded cheese evenly over the sausage and vegetables.
If using, sprinkle the bread cubes over the cheese layer.
Pour the egg mixture evenly over all the layers in the baking dish.
Bake uncovered for 30 to 40 minutes until the casserole is set in the center and the top is golden brown.
Remove from oven and let cool for 5 minutes before slicing.
Serve warm and enjoy this hearty and comforting meal any time of day.