In a large skillet, cook Italian sausage until brown. Drain off fat. Cut the crusts from the bread slices, if desired. Cut bread into cubes.
In a 12x7-1/2x2-inch baking dish, evenly spread half of the sausage. Top with bread cubes, then shredded mozzarella cheese. Sprinkle with remaining sausage.
In a mixing bowl combine eggs, milk, salt, and mustard. Pour into baking dish. Cover and refrigerate for 3 to 24 hours.
To serve, bake, covered, in a 325 degree F oven for 30 minutes. Uncover; bake for 15 to 20 minutes more or until a knife inserted near the center comes out clean. Let stand for 10 minutes. Makes 8 servings.