For the potato gratin, peel and wash the potatoes and slice them into thin slices. Put them into a greased gratin dish and season with salt, pepper and nutmeg. Pour cream evenly over it and bake in a preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 45 minutes until golden brown.
Wash the loach chops, cut into wide strips and sprinkle with lemon juice. Boil 1/2 litre salted water, 1/8 litre wine, peeled and diced onion. Put the fish in and let it simmer for about 10 minutes.
Boil up the rest of the wine, stir in the mustard and fold in the cold butter in flakes. Do not let it boil any more! Add capers and season the sauce with salt and pepper. Drain the fish and arrange on plates with the sauce and gratin.
Garnish with lemons and parsley. Serve with a fresh salad.