Wash potatoes and cook with skin in boiling water for about 25 minutes.
Meanwhile, for the sauce, peel and halve the onion and cut into strips. Wash the herbs and shake dry. Chop the dill finely and cut the chives into fine rolls. Wash and quarter the apple and remove the core. Cut apple quarters lengthwise in half, then crosswise into thin slices. Mix with vinegar. Season yoghurt with salt, pepper and sugar. Stir in onion, herbs and apple.
Wash the fish, dab dry and cut into 4 equal slices. Season with salt and pepper. Heat oil in a large pan. Fry the fish for about 4 minutes on each side.
Drain the potatoes, quench briefly and peel as desired. Serve the fish and potatoes with the sauce.