For the salad, wash the potatoes and cook in water for about 20 minutes. Then rinse with cold water, peel and let cool. Roast mustard seeds in a pan without fat for 1-2 minutes, remove. Peel and finely dice the onion.
Heat 2 tablespoons of oil in a saucepan. Fry the onion until transparent. Add 75 ml water and vinegar and simmer for 1-2 minutes. Pour everything into a bowl. Slice the potatoes directly into it, mix carefully and let it stand for about 30 minutes.
Cut the gherkins into slices. Wash and halve the tomatoes. Mix sour cream, mayonnaise, cucumber water and mustard seeds. Carefully fold the tomatoes and gherkins into the potatoes.
For the meatballs, wash the dill, shake dry and cut finely. Grind the bread finely in the universal chopper. Wash fish, dab dry and chop very finely. Mash fish, egg, dill, breadcrumbs and lemon juice with a potato masher to a coarse mass.
Season with about 1 tsp salt and pepper.
Form approx. 8 meatballs from the mixture. Heat 2 tablespoons of oil in a large frying pan. Fry the meatballs on each side over medium heat for 3-4 minutes until golden brown. Season the potato salad to taste again.
Dress everything.