In large pot, combine potatoes with enough salted water to cover. Place over medium-high heat; bring to boil. Cook until tender, 10-15 minutes. Drain; rinse under cold water until cool. Cut each potato in half; reserve.
In large bowl, combine buttermilk, sour cream, dill, vinegar, mustard, salt and pepper. Stir in potatoes, cucumber and onion until well coated. Refrigerate to allow flavors to blend before serving.