Peel, wash and halve the potatoes. Cook for about 10 minutes in boiling salted water, drain. Meanwhile clean, wash and cut the peppers into small pieces. Cut Leberkäse into small pieces as well.
Peel the onion and cut into slices. Heat 1 tablespoon of oil. Fry the potatoes and peppers for 5 minutes while turning. Add Leberkäse, fry for another 3 minutes and remove. Heat the rest of the oil in the pan.
Fry the onions and sauerkraut over a high heat for about 10 minutes and season with pepper. Mix all ingredients and fill into an ovenproof dish. Mix sour cream and mustard and spread on top. Cut cheese slices in half and place on top.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 10 minutes.