In pot, cover potatoes with 2 inches cold, salted water; bring to a boil. Cover and cook over medium until tender, about 8 minutes. Drain; cool. In bowl, mix sour cream, capers, mustard, lemon zest and juice; season. Spread half on platter. Top with potatoes. Dollop with remaining mixture. Top with radishes and watercress; season.
Cool Trick: When boiling spuds, start them in cold water to prevent them from being mushy on the outside and crunchy on the inside.