Wash the cured pork loin and pat dry. Place the meat on the fat pan of the oven. Peel onions, halve 1 onion. Add onion to the Kasseler with 1/2 litre of warm water. Cook meat in the preheated oven (electric cooker: 175 °C/ gas: level 2) for approx. 1 1/4 hours.
If necessary, add a little more water. Finely chop 4 onions and sauté in 2 tablespoons of hot oil until translucent, allow to cool. Dice the remaining onion and fry in the remaining oil. Add the sauerkraut and fry a little.
Deglaze with champagne and apple juice. Add bay leaves, juniper and peppercorns. Bring everything to the boil, cover and cook over a low heat for 45 minutes. Wash the grapes, possibly halve them. Add to the cabbage 40 minutes before the end of the cooking time.
Mix cooled onions, eggs, sugar, mustard and breadcrumbs. Season with salt and pepper. Remove the pork loin, spread the onion mixture on top and bake in a preheated oven (electric cooker: 200 °C/ gas: level 3) for a further 20 minutes.
Remove the roast and keep it warm. Pour broth through a sieve and fill up to 1/2 litre with water. Bring the stock to the boil, stir in the sauce thickener and bring to the boil again. Season to taste with salt and pepper.
Serve the roast pork with the champagne cabbage and sauce. Parsley and potatoes taste good with it.