Boil, drain, and mash potatoes. Add CHEESE to potatoes and beat with a mixer. Sauté onions in BUTTER. Add 1/3 of BUTTER and onions to potato mixture, add salt if desired. BUTTER a 13" X 9" baking dish and put a little water in the bottom. Layer noodles and potatoes, ending with noodles. Pour BUTTER and onions over all. Cover with foil and bake at 350º until heated thoroughly, about 20 mins.