Fry the fish fillets unfrozen in 1-2 tablespoons of hot oil for 13-15 minutes. Turn several times. In the meantime, peel and wash the potatoes and cut or slice them into thin slices. Fry in the remaining hot oil until golden brown, turning.
Season with salt and pepper. Grate the cheese for the sauce. Mix with sour cream and mustard and season with salt and pepper. Put potatoes and fish in an ovenproof dish. Spread the sauce over them. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 15-20 minutes.
Serve sprinkled with chopped parsley.