Recipe(tried): Broccoli Casserole

This is a favortite of ours - I make it with all my special family meals. Tastes very good the next day as well - if you have any left over :)
Broccoli Casserole
1/2 cup chopped onion
1/2 cup margarine
1/2 cup diced celery
1 cup Cheez Whiz (or 1 small jar)
1 cup cooked rice
1 can cream of mushroom soup
1 pkg of frozen broccoli, cooked and drained(our pkg. says 1kg= 2.2lbs - this can vary and I reallly use the winter mix with cauliflower and broccoli)
1 can of drained mushroom pieces
Saute chopped onion and celery in margarine. Add the msuhrooms, stirring until partially cooked. Add Cheez Whiz and stir until melted. Stir in mushroom soup, rice and brocolli. Pour into a casserole dish and bake at 350' for 30 minutes.