Remove crusts from bread. Cube 6 slices and spread on bottom of 9 inchx13 inch baking dish. Mix fish, celery, onion, chestnuts, mushrooms. Spread atop bread cubes, salt and pepper. Cube remaining bread and place on mixture. Mix beaten eggs, milk and mayonnaise. Pour over casserole and refrigerate overnight (covered). When ready to bake, remove from refrigerator. Cover with cream of chicken soup. Bake at 160ºC . for 45 minutes (uncovered). Remove from oven, cover with cheese. Bake 15 minutes longer. Excellent for a potluck or crowd. Good with crab, shrimp or halibut. Kathy, Haines, Alaska