Broccoli and Rice Casserole

1 C cooked rice
1 C chopped celery
1 C chopped onion
1 small jar cheese whiz
1 pkg frozen chopped broccoli
1/2 stick butter
1 can cream of mushroom soup
Salt and pepper
1 can sliced water chestnuts

Cook broccoli, onion and celery in small amount of water for 10 minutes. Drain and add other ingredients. Place in casserole pan and add chestnuts. Bake 30 minutes at 350F.