Put oven rack in middle position and preheat oven to 400°F.
Pat meat dry. Stir together allspice, 2 teaspoons salt, and 1 teaspoon pepper in a small bowl and rub all over meat.
Spread half of onions and half of garlic in a 13- by 9-inch roasting pan and arrange meat on top.
Spread remaining onions and garlic over meat. Tightly cover pan with foil and roast, turning meat over after 1 hour, until meat is very tender, about 2 1/2 hours total.
Skim fat from pan juices. Slice meat across the grain and sprinkle with parsley.
Serve with onions and pan juices.
Beef can be braised 2 days ahead. Cool meat in juices, uncovered, then slice and chill in juices, covered. Reheat, covered, in a 350°F oven 30 to 40 minutes.
