Fry the Kassler in a large roaster on the bacon side so that the fat is left out. Take the meat out of the pot and set it aside.
Chop the onions and sauté them in the fat, add the sugar and let them caramelize. Put the piece of meat back into the pot, pour the sauerkraut (with juice) over it. Spread a cup of cream over everything, then place the grapes on top. Close the lid and cook at about 160 - 180 degrees for 60 minutes. Finished!