In a small bowl, combine the lemon juice, oil and seasonings. Pour half of the marinade into a large resealable plastic bag; add lamb. Seal bag and turn to coat; refrigerate for at least 2 hours or overnight. Cover and refrigerate remaining marinade. Drain and discard marinade from lamb. Place lamb fat side up on a rack in a shallow roasting pan. Bake, uncovered, at 325° for 2 1/4 to 3 hours or until meat reaches desired doneness (for medium, a meat thermometer should read 160°; well-done, 170°), basting occasionally with reserved marinade. Let stand for 10-15 minutes before slicing.