Peel and wash the potatoes and cut them in half lengthwise. Heat the oil in a large pan. Fry the potatoes for about 15 minutes at medium heat. Season with salt and pepper. Clean, wash and cut spring onions into rings.
Mix yoghurt, mustard, starch and stock. Wash the fish, dab dry, cut into large pieces and pour lemon juice over them. Add yoghurt to the potatoes and bring to the boil while stirring. Place the fish on top and let it simmer covered at low heat for 8 minutes.
Wash the chives and cut into fine rolls. Sprinkle over the fish pan. Serve garnished with dill.