Chop the onion and make a marinade with mustard and ketchup, then season with salt and pepper. Cut the meat into slices, so don`t cut it all through, and spread the marinade between the slices. Carefully place the meat in a roaster and cover in the oven at 180 ° for 1 hour.
Then pour in the whipped cream and sour cream and cook uncovered for another 15 minutes.