Preheat oven to 350 degrees. Crumble and cook sausage in medium skillet over medium heat until browned. Drain off any drippings.
Spread bread cubes in greased 9 x 13 baking dish; mix sausage, peppers, mushrooms, onion, and spread on top of bread cubes. Layer cheese over all. Whisk eggs, milk, salt and mustard until well blended; pour over cheese evenly.
Bake 30-45 minutes or until toothpick comes out clean. Let stand 5 minutes before cutting into squares; serve hot. Refrigerate leftovers.