Wash the potatoes thoroughly and cook them with their skins in boiling water for about 15 minutes. In the meantime peel and finely dice the onion. Fry in 1 tbsp. hot oil. Add 6 tbsp. water, vinegar and stock, bring to the boil briefly.
Mix with salt, pepper, 1 teaspoon sugar and mustard. Fold in 3 tbsp. oil.
Drain the potatoes, quench and cut in half. Mix the potato halves well with the hot marinade and leave to soak for at least 1 hour.
Finely dice the cucumbers. Chop capers coarsely. Pluck trout filets into pieces. Wash dill, shake dry and chop finely. Add cucumber, capers, trout fillets and dill to the potato salad. Season again with salt and pepper.