1. Bring large pot of water to boiling. Scrub potatoes. Place in boiling water; cook until tender, about 20 minutes. Drain and cool. Cut the potatoes into 1-inch cubes. 2. To prepare the dressing: Stir together the sour cream, mustard, fresh dill, salt and pepper in a large bowl. 3. Add the potatoes to the dressing in the bowl; toss gently to coat the potatoes evenly with the dressing. Cover the bowl and refrigerate for at least 2 hours or until ready to serve. Garnish with fresh dill sprigs if you wish.