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.
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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.