Saute the onions in the butter. Stir in the 1-1/2 cups dressing, soup, and sour cream. Pour the ingredients into a 9 by 13 inch greased pan. Saute the remaining 1 cup dressing in 2 T. butter. Sprinkle the topping on top of the casserole. Bake at 350 degrees for 30-40 minutes. Serves 6-8.