Boil the potatoes in their skins and let them cool. Peel the potatoes and cut into medium-fine cubes. Peel, quarter, core and dice apples. Finely dice the beetroot, pickles and onions as well. Mix the yogurt, mayonnaise, vinegar, mustard and the spices into a sauce. Mix the sauce with the prepared salad ingredients. Wash and cut the cress and stir into the salad.