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.