Lift the joint of meat out of its pack, cut off the strings and discard the added piece of roasting fat. Make a horizontal cut across the middle of the joint, without cutting through, then fold back the two sides to create one large, flat piece. Place the meat in a large, shallow dish. Make a marinade by mixing together the tarragon, pepper, mustard and olive oil then massage all over the meat. Leave to marinate in the fridge for 1½ hours, then rub in the vinegar and set aside for a further 20 minutes. Cook on a medium-heat barbecue for 15 minutes on each side then rest for 10 minutes before carving. If you're barbecuing for lots of people, cook the meat in the oven first then finish it on the barbecue for flavour and to ensure that it is thoroughly cooked. This is particularly important for burgers, sausages, pork and poultry.
