Heat some oil in a large saucepan. Fry the meat bones and the halved onions with the cut surface facing down. The onions can look almost black. This will give the sauce its color later.
Roast the roughly chopped soup vegetables as well as the tomato paste. When everything has taken on a nice color, pour about 2 liters of water. Add the cloves and bay leaves and put the well-washed tongue in the broth. First of all, don't add salt. The cured tongue is sometimes more, sometimes less salty. Try the broth after half an hour of cooking. Possibly add salt. The whole thing has to cook for a total of 5 to 3 hours.
If the tip is easy to pierce, the tongue is cooked. The skin is then quite loose at the tip.
Remove the tongue from the broth, let it cool down a little, and then carefully peel off the skin. It's very easy now. Cut off excess fat and connective tissue. Return the tongue to the broth and refrigerate overnight.