Peel and boil the potatoes. When cool enough to handle, slice them. Set aside. Slice the onions and lightly saute them. Layer the potatoes and onions, alternately, in a buttered 9 x 13 baking dish. Mix together, in a pot, the butter, soup, sour cream and cheese. Stir and cook over low heat until the cheese melts. Pour all of this mixture over the top of the potatoes/onions in the baking dish. Sprinkle the breadcrumbs/cornflakes over the top of the soup mixture. Bake, uncovered, at 350 until hot (about 45 minutes from room temperature or about 1 hour from frig temperature). NB - this dish can be made one day ahead, stored in the refrigerator and heated just before the dinner is served.