Combine the olive oil, lemon juice, onion, parsley and seasonings.
Cut slits in the lamb and rub the marinade into the meat.
Wrap the lamb in heavy aluminum foil and marinate overnight in the refrigerator.
Preheat the oven to 325 degrees.
Open the foil and place the lamb in a roasting pan.
Roast one and one-quarter hours, using the marinade and pan drippings for basting.
For rare lamb a meat thermometer should register 140 degrees.
If a well done roast is desired, bake thirty minutes longer, or until thermometer registers 170 degrees.