Preheat the oven to 160*C. Cut about 18 small slits randomly in the meat. Insert the mint into the slits. Rub the surface of the meat with lemon juice. Sprinkle with the parsley and onion. Pat into the meat. Place the meat, fat side up, on a rack in a roasting pan. Insert a roasting thermometer in the centre of the thickest portion without touching the bone. Roast for 1 1/2 to 2 hours or until thermometer registers 140*C for medium rare or for 2 to 2 1/2 hours or until 155*C registers for medium. Cover the meat with foil and let stand for 15 minutes before carving. Meanwhile, in a small saucepan, combine the mint jelly, cornstarch and lemon rind. Stir in apple juice. Cook over moderate heat, stirring constantly, until mixture starts to thicken. Cook and stir for 2 minutes more or until jelly melts and mixture is thickened. Serve with the meat.