Prepare the Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter.
Prepare the Bread: Spread the cubed bread evenly in the baking dish.
Layer the Ingredients: Sprinkle the cooked sausage over the bread cubes. Add diced bell peppers, onions, and tomatoes, distributing them evenly.
Whisk the Eggs: In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, and dried mustard until well combined.
Pour the Egg Mixture: Pour the egg mixture evenly over the ingredients in the baking dish. Ensure that the liquid covers the bread and other components.
Add Cheese: Sprinkle the shredded cheddar cheese over the top of the casserole, providing a deliciously cheesy layer.
Allow to Soak: Let the casserole sit for about 10-15 minutes, allowing the bread to absorb the egg mixture.
Bake in the Oven: Place the baking dish in the preheated oven and bake for 45-50 minutes or until the top is golden brown and the center is set.
Check for Doneness: Insert a knife or toothpick into the center; if it comes out clean, the casserole is done.
Serve Warm: Allow the casserole to cool for a few minutes before slicing. Serve warm, and enjoy a hearty, flavorful breakfast!