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.
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