Peel and wash the potatoes. Cut into cubes of about 1 cm. Cook in salted water for 5 minutes, drain. Cut Kasseler also into 1 cm cubes. Wash, quarter, core and dice apples.
Whisk egg, sour cream and mustard. Season with salt and pepper. Grease a suitable casserole dish or box form. Layer potatoes, cured pork loin, apples and sauerkraut alternately. Spread the sour cream mixture over it.
Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes. Arrange on plates. Serve garnished with some salad leaves and apple slices.