Cook potatoes in salted water. Cool and peel. Fry bacon until crisp. Remove from drippings. Fry onion rings until soft. Remove from drippings. (Make sure there is still 1/3 cup left in the pan.) Combine water, vinegar and egg yolk. Add to drippings, stirring constantly. Add sugar; stir until dissolved. Cook until thick and bubbly. Pour over potatoes, onions and bacon. Bake about 20 minutes at 350°, or until heated throughout. Serves 6 to 8.