Cover potatoes with cold water, bring to a boil and salt the water. Boil potatoes until just tender, 12 to 15 minutes. Drain.
Meanwhile, combine the honey, Dijon, and vinegar in a bowl then slowly whisk in the olive oil. Season with salt and pepper. To the bowl add radishes, capers, onions, celery, watercress or parsley and hot potatoes. Using a wooden spoon break up the potatoes into large chunks. Toss the potato salad, adjust salt and pepper then serve.