If necessary, wash the potatoes and cook with the skin on. Peel still warm. Then cut into slices.
Peel and dice the onions. Clean and finely dice May turnips. Sauté onions and turnips in oil. Pour on the cucumber stock. Add mustard and cook until soft. Now season well and add to the potatoes. Season to taste and let it steep (I think at least an hour or better overnight). Stir in the sour cream before serving and season again to taste.