Preheat oven at 400. Prepare a pie pan with cooking spray. In a mixing bowl, combine turkey, peppers, and onions. Spread mixture into prepared pan. In another mixing bowl, combine egg whites, milk, flour, salt, and pepper. Pour over turkey mixture. Bake for 25 minutes or until browned on top. Sprinkle with cheese and bake for 3 minutes more or until cheese is melted.