Mix cream of chicken soup, cooked chicken, celery, 1 cup cornflakes, water chestnuts, mayonnaise, and onion together in a large bowl; spread mixture into a 9x13-inch casserole dish.
Mix 1 cup cornflakes and melted butter together in a bowl until coated; sprinkle over casserole.
Bake in the preheated oven until casserole is bubbling and topping is crisp, about 45 minutes.
I used Honey Bunches of Oats with Almonds(R) cereal instead of cornflakes. I also tripled the chopped celery instead of using a can of water chestnuts. I used reduced fat cream of chicken soup and Hellman's(R) lowfat mayo. I used leftover chicken from a rotisserie I had purchased earlier in the week. Delicious!