Marinate the meat
Place the meat in a large bowl. Pour over the wine and vinegar, stud the onion with bay leaf and clove, add with the pepper. Chop the carrots, slice the leek into rings and add with the thyme. Cover and marinate the meat in the fridge for about 4 days, turning daily.
Remove the meat and vegetables, pat dry. Bring the liquid to the boil, strain through a sieve, set aside. Heat the clarified butter in a pan. Salt the meat and brown for approx. 20 mins. on each side. Remove the meat. Sauté the vegetables in the same pan, adding around 500 ml of the reserved liquid, bring to the boil. Return the meat to the pan. Braise, covered, over a very low heat for approx. 2 hrs. Turn the meat occasionally, adding a little liquid if necessary.
Sauce & carving
Remove the meat, cover and leave to stand for approx. 10 mins. before carving. Pour the sauce into a small pan through a sieve and reduce by half. Stir the cornflour into the wine, add while stirring, simmer the sauce for approx. 5 mins., season the sauce. Carve the meat across the grain and serve with the sauce and vegetables.