Wash potatoes thoroughly and boil in water for about 20 minutes. Clean, wash and shake dry lamb's lettuce. Clean and wash radishes. Carve a "toadstool" from each radish.
Drain the potatoes, rinse with cold water and peel. Chop the potatoes and mash them finely. Peel onions, dice finely and add to potatoes. Stir in vinegar. Season with salt and pepper. Fold in the oil. Season dressing to taste. Arrange the salad with the radish mushrooms and sprinkle with the dressing.
Peel the lower third of the radishes so that a small mushroom stalk is formed (use sections for the salad!). Cut several white dots into the red mushroom hat with a sharp knife.