Steam the Kasseler in a little white wine (do not boil). A small piece takes about 20 to 25 minutes.
Clean the leek and cut into rings. Wash the peppers and cut into small strips. Clean the mushrooms and cut them into leaves.
Fry the leek in margarine or oil. Add peppers and mushrooms. Briefly sauté. Fill up with the stock and the white wine in which the Kasseler was stewed and stew for about 10 minutes. Add tomato paste and season with salt, pepper and thyme (be careful with salt, as the smoked pork is salty). Cut the meat into slices and place in a baking dish. Spread the sauce over it, sprinkle cheese over it.
Bake in a hot oven at about 220 ° C top / bottom heat until the cheese is golden yellow.