Peel and wash the potatoes and cook for about 20 minutes.
Peel the onion and cut into cubes. Wash, shake off, chop the herbs and set aside a small portion for garnish. Clean the apple, quarter it and remove the core, then dice the apple and drizzle with vinegar.
Mix the yoghurt with the apple pieces, herbs, onion cubes and spices and put in a cool place.
Wash the cod fillets, pat dry and season with salt and pepper. Heat the oil and fry the fillets on both sides for about 4 minutes.
Drain the potatoes and serve with the fish and yogurt sauce. Garnish with the remaining herbs.