Cook sliced potatoes, uncovered, in salted boiling water for about 12 minutes, or until tender. Drain well; spread on a baking pan lined with parchment paper to cool.
Meanwhile, in large mixing bowl, stir together salad dressing, sugar, half-and-half, mustard and pepper.
Add cooled potatoes to dressing mixture, along with the celery, radishes and onion. (If you like, reserve some celery and radishes to use as a garnish.) Stir gently to combine. Chill, covered, up to 4 hours. If you like, garnish with reserved radish and celery.
