Preheat oven to 400 degrees. In a blender, combine the oil, garlic, 1 tablespoon salt, and 1 teaspoon pepper. Blend until smooth and emulsified, scraping down the sides as needed.
Place the lamb, fat side up, on a rack set in a roasting pan; spread garlic paste all over lamb. Roast in oven until a meat thermometer inserted into the thickest part of the meat registers 135 degrees, for medium rare, about 2 hours.
Transfer lamb to a platter, and discard string; keep warm while you make the pan sauce. Pour wine or 1 cup water into the roasting pan in which the lamb was cooked. Place over medium heat, and scrape up browned bits from bottom of pan. Add stock and 1/2 teaspoon salt, and bring to a simmer. Cook, stirring, until liquid has reduced to 1 1/2 cups, about 10 minutes. Skim fat from surface, as needed.
Slice lamb just before serving. Strain sauce into a gravy boat, serve hot with lamb.