To finish the beef, get a barbecue fired up and wait until the coals are completely ashen Ã¢â‚¬â€œ you want a low, not fierce, heat. Lift the beef out of the roasting tin and place on the barbecue to char. If your barbecue has a lid, close it now. Cook for about 20 mins, turning it carefully with tongs, until lightly charred all over and heated through. Lift onto a board and serve sliced with the remaining barbecue sauce on the side.