Place lamb in a large shallow, no reactive dish. Combine crushed peppercorns in a small bowl; mixing well. Set aside 2 tbsp. crushed peppercorn mixture. Combine remaining peppercorn mixture, mint leaves, Burgundy, vinegar, garlic, and rosemary; mixing well. Pour marinade mixture over lamb. Cover and marinate in refrigerator for 8 hours or overnight, turning occasionally. Remove lamb from marinade, reserving marinade. Roll lamb, tying with heavy string at 2-inch intervals. Coat lamb with mustard; lightly pat reserved peppercorn mixture into mustard. Place lamb in a shallow roasting pan; pour reserved marinade around lamb. Insert meat thermometer into thickest part of lamb. Bake in a preheated 350 degrees oven for 2 hours and 15 minutes or until meat thermometer registers 160 degrees basting occasionally with pan juices. Let stand for 20 minutes before slicing. Serve with pan juices.