Wash the potatoes and boil in water for about 20 minutes. For the marinade peel and dice the onions. Heat the oil in a pan and fry the onions until transparent, turning them over. Sprinkle with sugar and lightly caramelise. Stir in vinegar and mustard. Season well with salt and pepper. Simmer at low heat for about 10 minutes.
Drain, quench, peel and slice the potatoes. Mix carefully with the hot marinade in a large bowl. Leave to stand for at least 2 hours.
Clean and wash the lamb's lettuce and shake it well dry. Clean, wash and slice the radishes. Cut sausage into sticks. Wash chives and cut into fine rolls. Carefully fold everything into the potatoes. Season with salt and pepper.
Tip: For more flavour, mix sausage and potatoes with the marinade the day before. Only fold in lamb's lettuce and radishes before serving, then it will remain crisp.