I've made this countless times for my husband, sons, and friends. The whole house smells fabulous while its in the oven. Meat is beyond tender!! You'll want lots of crusty bread on hand to soak up the juices.
Put oven rack in middle position and preheat oven to 400 degrees.
Pat meat dry. Stir together allspice, 2 tsp salt, and 1 tsp 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 heavy-duty foil and roast, turning meat over after 1 hour, until meat is very tender, about 2 to 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.