Boil the potatoes in their skins, then peel and slice them into a serving bowl while still hot and steaming. Fry the bacon until crisp, remove from skillet and set aside. Add to bacon drippings the vinegar, water, sugar, mustard; bring to boil. Add the onion to the hot sauce for a very short time, to gentle it while retaining most of its crispness. Add parsley, salt and pepper to the potatoes, pour the sauce over them, and toss with a fork. Serve hot or warm. (Or can be eaten cold, if you don't object to congealed bacon fat!)