Trim fat from lamb. Put small slits in surface of lamb; insert garlic slivers. Place lamb, fat side up, on a rack in a shallow roasting pan. Insert a meat thermometer. Roast in preheated oven at 325°F for 15 to 20 minutes per pound or until meat thermometer registers 145°F to 150°F for medium-rare.
Meanwhile, in a small saucepan stir together jelly, lemon peel, lemon juice, gingerroot, salt, mustard and pepper. Heat through; keep warm. Brush roast with jelly mixture several times during the last 30 minutes of cooking. Let roast stand for 15 to 20 minutes before carving.