Steam broccoli and onion together until tender but still bright in color, and drain well. Combine condensed cream of mushroom soup, mayonnaise, cheese, broccoli, and onion, and spoon into a greased casserole dish. Crush cheese nips crackers, and combine crumbs with melted butter; sprinkle over top of casserole. Bake at 350 for 45 minutes or until bubbling through. Serves 6. This recipe is from Aunt Michelle.