Make several slits on outside of lamb; insert garlic slivers. Combine mint, salt, pepper and thyme; run on roast. Layer onion in roasting pan; place roast on onions. Insert meat thermometer, making sure it does not touch fat or bone. Bake at 325° 2 to 2 1/2 hours. Combine sherry and jelly. Glaze roast with sherry mixture; bake additional 30 minutes or until thermometer registers 160°. Let stand 10 minutes before carving.