Preheat the oven to 350˚F.
Spray a 13 by 9-inch baking dish with nonstick cooking spray.
In a large skillet, cook the sausage over medium heat, stirring frequently, until browned and crumbly, about 10 minutes; drain well on paper towels.
Cut and discard the crusts from the bread. Cut the slices in half, and arrange in a single layer in the prepared baking dish, cutting pieces to fit as necessary to cover the bottom of the dish.
Sprinkle with the sausage and cheese.
In a large bowl, whisk together the eggs, milk, mustard, salt, and seasoned pepper; carefully pour the mixture over the cheese.
Bake casserole until set and golden, about 40 minutes. Let stand for 10 minutes before serving.