Wash the potatoes and cook in boiling salted water for about 20 minutes. Then peel and let cool. In the meantime peel and chop the onions. Clean, wash and drain the lamb's lettuce.
Cut the potatoes into slices. Bring vinegar and onions to the boil. Season with salt, pepper and sugar. Carefully mix with the potatoes while still hot. Cover and leave to stand for about 30 minutes. Then fold in lamb's lettuce and oil, season to taste again and serve.