Wash the potatoes and cook in salted water for 20-25 minutes. In the meantime peel and finely dice the onions. Cut cucumbers into strips. Wash apples, quarter them, remove seeds and cut into slices.
Bring vinegar, goose fat, sugar and stock to the boil. Cook onions, cucumbers and apples for about 2 minutes. Drain and peel the potatoes, let them cool down a bit and cut them into slices. Pour the onion-cucumber-apple-mixture with the liquid over the potatoes.
Leave to stand for 20 minutes. Turn carefully every now and then. In the meantime, soak the rolls in cold water. Mix minced meat with egg and the squeezed out roll. Season with salt, onion pepper and mustard and form 8 meatballs with moistened hands.
Heat the oil in a frying pan. Fry the meatballs for about 5 minutes on each side. Wash the chives and cut into small rolls. Season potato salad with salt and white pepper. Sprinkle with chives and capers and serve with the meatballs.