Add a splash of oil to the pan and fry the beef for a couple of minutes until it is browned on each side. Add the red wine and let it bubble until reduced a bit, then add the tomato puree, pickled onions and stock. Bring to the boil, cover and then transfer to the oven. Cook for 90 minutes or until the beef is tender to the touch. Drain the meat in a sieve over a bowl and keep the liquid. Allow to cool.