First, prepare the lamb. Spread the boned leg of lamb flat on a work surface with the cut side of the meat facing upwards. Cut out the membranes and tendons and discard them. Starting from the centre of the meat, slice horizontally into the flesh at one side of the leg, making sure that you do not cut completely through the meat. Open out the resulting flap. Then slice into the opposite side of the leg and open it out in a similar manner. The meat should be no more than 5 cm (2 inches) thick if you plan to barbecue the lamb. To grill the meat, preheat the grill for 10 minutes.
To make the chutney glaze, combine the stock and the mango chutney in a small saucepan and bring the mixture to a simmer over medium heat. Stir the mustard and the cider vinegar into the cornflour paste and then whisk this mixture into the simmering stock and chutney. Cook the glaze, stirring continuously, until it thickens about 1 minute. Remove the pan from the heat and set the glare aside
With your fingers, rub both sides of the lamb with the safflower oil. Grill the lamb. turning it every 5 minutes, until it is well browned on both sides about 70 minutes in all. Sprinkle the salt and some freshly ground black pepper on the lamb and brush it with some of the chutney glare Continue cooking the lamb, turning and basting it frequently with the glare. for about another 10 minutes Pour the remaining chutney glare into a small serving bowl.
Transfer the lamb to a cutting board and sprinkle it with the chopped mint. Allow the meat to stand for 10 minutes before carving it into slices. Arrange the slices of lamb on a warm serving platter and serve accompanied by the remaining chutney glaze and garnished with mint sprigs.
