Spray a 13" x 9" casserole dish w/ cooking spray. In bowl, blend together egg,milk, onion salt and dry mustard.
Place 8 slices bread in bottom of casserole dish. Place 8 slices swiss cheese over bread. Place 8 slices ham over swiss cheese. Place 8 slices american cheese over ham. Repeat the layers. Pour egg mix over ingredients. Cover w/ foil. Refrigerate overnight.
The next morning, heat oven to 350. Uncover casserole and sprinkle w/ cornflakes. Drizzle w/ butter. Bake 1 hr.