For the cucumber relish drain the cucumbers. Peel onions. Wash, quarter and core the apple. Dice everything finely. Peel and grate horseradish. Heat 1 tbsp. oil in a saucepan. Approx. 2⁄3 Sauté onions in it.
Stir in sugar and caramelise slightly. Add cucumber, apple, horseradish and vinegar. Season with salt and pepper. Simmer over low heat for about 15 minutes. Let it cool down.
Wash the potatoes thoroughly and boil them covered in water for about 20 minutes.
For the meatballs roast mustard seeds and let them cool down. Knead minced meat, breadcrumbs, egg, mustard seeds and the rest of the onions. Season with salt and pepper. Form approx. 12 small meatballs from the minced meat mixture.
Heat 2 tablespoons of oil in a large frying pan. Fry the meatballs in it for about 4 minutes on each side until golden brown.
Wash the herbs, shake dry, chop and stir into the relish. Season quark with salt and pepper. Drain potatoes, rinse and peel. Serve with meatballs, cucumber relish and quark.
Drizzle quark with linseed oil.