Wash, scrub, and quarter the potatoes but do not peel. Boil until just done, then drain and chill. (This is best done the night before, for a breakfast meal.) Heat butter or oil in a large frying pan over medium heat. Saute the onion and pepper until just sizzling. Cut the potatoes into bite-size chunks and add to the pan. Stir-fry until potatoes are golden. Season and serve hot.