Preheat oven to 350F. Saute onions and mushrooms in butter until clear. Add soup, cheese and garlic powder, stirring until cheese melts.
Add broccoli, rice and ham, and mix well. Gently stir in pimientos and spoon mixture into greased baking dish. Cover and chill for 8 hours.
To bake, remove from refrigerator and let stand for 30 minutes. Bake covered at 350F for 40 minutes. Sprinkle onion rings over casserole and bake 5 minutes.