Preheat a skillet over medium heat. Add the sausage, chopped onion, and bell pepper. Break the sausage into small pieces and cook until browned and the vegetables are softened, about 6 to 8 minutes. Drain excess fat if needed.

In a large bowl, whisk together the eggs, milk, mustard if using, and a pinch each of salt and pepper until smooth.

Grease a 9x13-inch baking dish with butter or nonstick spray. Spread the cubed bread in an even layer across the dish.

Scatter the cooked sausage and vegetables evenly over the bread. Press lightly so cubes settle.

Pour the egg and milk mixture over the bread and sausage. Use a spatula or your hands to press down so most bread pieces are moistened. Let sit for 5 minutes to help absorption.

Sprinkle the shredded cheese evenly on top. Cover the dish tightly with plastic wrap or foil and refrigerate overnight or at least 6 hours.

The next morning, remove the cover and preheat your oven to 350°F (175°C).

Bake the casserole uncovered for 30 to 40 minutes, until the top is golden and the center is set. An instant-read thermometer should register about 160°F in the center.

Let it cool for 5 minutes before slicing and serving.