Melt 2 tablespoons of butter in a heavy skillet. When foam subsides add the bacon. Fry 5 to 10 minutes or until the bacon is lightly browned. Remove from skillet and set on paper towels to drain. Fry the onion slices for 6 to 8 minutes in the fat remaining in the skillet, adding more butter as necessary. When the onions are soft and transparent, add the apple rings and cover the pan. Simmer over low heat for 5 to 10 minutes, shaking the pan gently at intervals to prevent the apples from sticking. When the apple rings are sufficiently cooked (they should offer little or no resistence when pierced with the tip of a sharp knife or skewer), return the drained bacon to the skillet. Cover and simmer an additional 3 to 5 minutes or until the bacon is heated through. Grind fresh black pepper liberally over the top and serve directly from the pan as a luncheon entr, e or Sunday night supper.