Wash the potatoes and cook covered for about 20 minutes. Quench, peel and let cool down.
Onions peel and chop finely. Bring to the boil with 200 ml water, stock, vinegar, a little salt, pepper and 1 teaspoon sugar. Cut the potatoes into slices. Pour the hot marinade over them in a large bowl.
Leave covered for at least 1 hour.
Clean, wash and chop the radishes. Wash cucumber and chives. Peel cucumber, if necessary, cut into quarters lengthwise and cut into pieces, chives finely.
Stir remoulade and soured milk until smooth. Add cucumber, radishes and chives to the potatoes. Leave to stand for at least 1 hour. Season salad again.