Melt margarine in skillet and saute onions until tender. Cook and drain noodles. Prepare mashed potatoes according to box directions for 10 servings. Mix margarine, onions, and noodles together in 9 x 13 baking dish. Spoon potatoes over top and spread them evenly. Sprinkle with cheddar cheese. Cover and bake at 350 for about 25 min. Until hot and "bubbly."