Place 6 slices of the toasted bread in the bottom of a lightly greased 9x13 inch baking dish. Sprinkle the chicken, celery, and onion on top of the bread, then cover with the other 6 slices of toasted bread.
In a medium bowl mix the egg substitute, mayonnaise, milk, and salt and pepper. Pour over the sandwiches, then top with soup. Cover baking dish with aluminum foil, and refrigerate 8 hours, or overnight.
Preheat oven to 325 degrees F (165 degrees C).
Bake casserole in preheated oven for 45 minutes. Remove cover, sprinkle with cheese, and continue baking 30 minutes. Allow to sit 10 minutes before serving.
The original recipe, Chicken Sandwich Casserole, was submitted by Teri Denlinger, and contains 591 calories, 39 g fat, 198 mg cholesterol, 858 mg sodium, 26.7 g carbohydrates, 1.2 g fiber, and 32.4 g protein.