If desired, add the one third cup milk, quarter cup butter, salt & pepper to use potato centers as mashed potatoes. When ready to prepare skins, melt the one third cup butter. Brush shells with the butter, inside and out. Place, cut side up, on baking sheet. Bake in a 500 degree oven for 20 minutes or until brown and crisp. Distribute some bacon, onion, and cheese in each shell. Bake just until cheese melts. Serve hot. If the potatoes are large, you may want to cut in half crosswise before serving. You may want to double this recipe if you are serving several people, especially if they are hungry!