Peel and wash the potatoes and cut them into thin slices. Grease a gratin dish and layer the potatoes. Season with salt, pepper and nutmeg. Pour milk over it and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 40 minutes.
In the meantime, clean and wash the carrots and cut them into fine slices. Steam in a little salted water for about 10 minutes. Heat 1 teaspoon of oil in a frying pan and sauté the carrots. Peel the horseradish, slice very thinly and add to the carrots.
Season steaks with pepper and fry them in a pan with hot oil for 5 minutes on each side. Season with salt. Stir mustard and sour cream until smooth. Place 1 teaspoon of each on the steaks. Arrange the steaks with the vegetables and the gratin on plates.
Garnish with parsley.