Put the dumplings in a pot of cold salted water and bring to the boil. Let it boil for about 1 minute. Let it simmer in an open pot at low heat for about 15 minutes. In the meantime, peel onion and cut into slices.
Steam in 1 teaspoon of hot fat for about 3 minutes. Add red cabbage and braise for about 5 minutes. Remove the smoked pork from the bone and cut into cubes. Fry in remaining fat at medium heat for about 5 minutes.
Deglaze with 50 ml water and bring to the boil. Season to taste with mustard, salt and pepper. Wash the thyme, dab dry and pluck the leaves from the stalks. Add to the sauce. Quench potato dumplings cold and remove from the cooking bag.
Remove the cook bag. Arrange the dumplings with the red cabbage and the Kasseler sauce on a plate. Sprinkle with pepper and serve garnished with parsley and thyme as desired.