Drain the cucumbers and cut them into small cubes. Peel and finely dice the onions. Wash, quarter, core and dice the apple. Peel horseradish and grate finely. Heat 2 tablespoons of oil in a saucepan. Fry about 2/3 of the onions in it. Add sugar and caramelise slightly. Add cucumber, apple and horseradish. Deglaze with vinegar and season with salt and pepper. Let simmer at mild heat for 12-15 minutes. Let cool off
Roast mustard seed in a pan without fat. Take out and let it cool down. Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Knead minced mustard seed, remaining onion, breadcrumbs and egg. Season with 1 1/2 tsp salt and pepper. Form 12 meatballs from the minced meat mixture. Heat 2 tablespoons of oil in a large frying pan. Fry the meatballs on each side for about 5 minutes
Wash the herbs, dab dry. Pluck off leaves or flags and chop finely. Mix the herbs, except for a little bit for sprinkling, into the relish. Stir curd cheese until smooth, season with salt and pepper. Drain potatoes, rinse with cold water and peel. Arrange potatoes with relish, quark and meatballs. Sprinkle quark with linseed oil and sprinkle everything with remaining herbs