Melt the butter in a saucepan and fry the chopped onion till it begins to brown. Sprinkle with a little flour, and when this browns add the hot stock gradually, and finally the vinegar, lemon peel, bayleaf and a seasoning of salt and pepper. Simmer for ½ an hour. Add the hot boiled potatoes, cut into small cubes, and simmer for 5 minutes. Remove the bayleaf and lemon peel, and serve.