Cook one day before serving!
Cut onions into rings, pile each meat and onion rings on edge (against each other) in a roasting pan (not on top of each other), pour the bottle of tomato ketchup over the meat and leave for one night. Bake in the oven at 200 °C for 2 hours.
Tip: If you want you can pour 1/2 bottle of beer over it 1/2 hour before the end of the cooking time.