Preheat oven to 325°. Combine the oil, rosemary, onion, garlic, salt and pepper; rub over lamb. Place fat side up on a rack in a shallow roasting pan.
Bake, uncovered, 2 to 2-1/2 hours or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), basting occasionally with pan juices. Let stand 15 minutes before slicing.