In a large bowl, combine the potatoes, celery, onions and radishes. Combine the sour cream, vinegar, salad dressing mix and pepper; pour over potato mixture and toss gently. Press into a 5-cup ring mold coated with cooking spray. Cover with plastic wrap. Refrigerate for 2-3 hours.
Run a knife around edges of mold to loosen. Invert onto a serving plate. Garnish with Red Radish Rosettes if desired.