In a large skillet, cook the sausage, green peppers and onion over medium heat until meat is no longer pink; drain. Meanwhile, in a large bowl, whisk the eggs, milk, salt, pepper and mustard. Stir in the sausage mixture, bread, spinach, cheeses and zucchini.
Transfer to a greased 13-in. x 9-in. baking dish. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Cover and bake at 350° for 40 minutes. Uncover; bake 40-45 minutes longer or until a knife inserted near the center comes out clean. Yield: 10-12 servings.