Spray an 11x7x1-1/2-inch baking dish (2-quart rectangular) with some cooking spray.
Cut each slice of bread diagonally into quarters. Arrange in prepared dish. Sprinkle the onion and cheeses evenly over the bread.
In a large mixing bowl, combine eggs, milk, mustard, Italian seasoning, and red pepper. Pour slowly over the bread-cheese mixture. Press lightly with back of spoon or spatula to moisten bread. Cover with foil and refrigerate for 2 to 24 hours.
Bake foil-covered strata in a 325 degree F oven 45 minutes. Uncover and bake about 10 minutes or until set and a knife inserted off center comes out clean. Let stand for 10 to 15 minutes before cutting. Serve with salsa and sour cream, if you like.