Grandma Ethel's Chicken Broccoli Casserole

1 stick butter
2 packages frozen chopped broccoli (I've used fresh many times too)
1/2 pound jar cheese whiz (yes, I know this isn't really a food is it?)
1 cup chopped onion, medium
1 can mushroom soup
1 cup cooked rice
1 chicken, cooked, deboned and cut up

Thaw broccoli (if using frozen), saute it with butter and onion in a skillet. Add other ingredients. Stir often after each addition. Pour into 2 quart casserole dish. Bake 350 for 30 minutes. Add alittle water/milk if it looks dry.