Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish.
Cook sausage in a skillet over medium heat until no longer pink (5-7 minutes).
Add onions and bell peppers to the skillet and cook for 3-4 minutes until softened.
Add garlic and cook for 30 seconds until fragrant. Remove from heat and drain excess fat.
Spread bread cubes evenly in the prepared baking dish.
Sprinkle half the cheese over the bread layer.
Spread sausage mixture evenly over the cheese.
In a bowl, whisk together eggs, milk, salt, pepper, mustard powder, and paprika.
Pour egg mixture evenly over the layers in the baking dish.
Top with remaining cheese.
Bake uncovered for 45-50 minutes until center is set and top is golden brown.
Let rest 5-10 minutes before serving.