Defrost the puff pastry. Peel and wash the potatoes and cut them into thin slices. Heat oil in a large pan and fry the pork loin over medium heat for 15-20 minutes, turning occasionally.
In the meantime clean the savoy cabbage, remove 4-5 beautiful savoy cabbage leaves from the stalk and wash them. Blanch leaves in boiling salted water for 3-4 minutes, then rinse under cold water and dab dry. Cut out thick savoy cabbage panicles.
Halve the garlic. Spread garlic on the gratin dish, grease it and layer potatoes in flakes. Season 350 g cream with salt, pepper and nutmeg and pour over the potatoes. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 45 minutes.
Remove the cured pork from the pan. Deglaze the roast with stock and set aside. Roll out the puff pastry rectangularly (12 x 22 cm). Cut the puff pastry with a ribbed pastry wheel into strips of about 1 cm wide.
Coat the Kasseler with mustard and fold over the savoy cabbage leaves. Place puff pastry strips on top of the savoy cabbage and brush with 1 tablespoon of cream. Bake the Kasseler in a casserole dish in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 35-40 minutes.
If the surface becomes too dark, cover with aluminium foil. Remove potato gratin from the oven and keep warm. Stir starch and 2-3 tablespoons of cold water until smooth. Bring to the boil and thicken with the starch.
Arrange the cured pork loin and sauce on a plate. Add gratin.