Combine first 6 ingredients in a blender or food processor; process until mixture forms a paste.
Trim fat from lamb, and place lamb in a shallow dish. Rub parsley mixture over lamb. Cover and marinate in refrigerator 8 hours.
Preheat oven to 325°.
Place lamb on a rack in a roasting pan coated with cooking spray. Insert a meat thermometer into thickest part of lamb, if desired. Bake, uncovered, at 325° for 1 hour and 45 minutes or until meat thermometer registers 145° (medium-rare) or desired degree of doneness.
Transfer lamb to a serving platter. Let stand 15 minutes before serving.