Peel and chop the onion. Peel, wash and thinly slice the potatoes. Heat 2 tablespoons of oil in a wide saucepan. Fry the diced onion until transparent. Add the potatoes.
Deglaze potatoes with 400 ml water. Bring to the boil, add stock, 1⁄2-1 teaspoon salt and a little pepper and simmer covered for about 10 minutes. Form 8 meatballs from the mince and fry in 1 tbsp. hot oil for about 8 minutes.
Clean, wash and slice the radishes. Clean, wash and slice the cucumber. Rinse cress if necessary and cut from the bed.
Fill potatoes in a bowl and season with vinegar. Fold in radish, cucumber, sour cream and cress. Serve lukewarm. Add the meatballs.