Preheat oven to 400°
Pat lamb dry with paper towels. Using a sharp knife, score the top side of lamb by making shallow cuts all over.
In a small bowl, combine diced garlic, olive oil, rosemary, all spice, salt, and pepper.
Place lamb, fat side up on a nonstick roasting pan with a nonstick roasting rack. If you don't have that, a large aluminum foil baking pan would work perfect too.Spread the garlic mixture evenly over the lamb. Make sure you rub the mixture thoroughly into the scored cuts. This will add flavor into the meat inside.Add 2 cups of water on the bottom of the pan. Doing so, will let the juice of the lamb fall in the water and will make the best lamb infused broth ever. On the bottom of the pan, add your cinnamon sticks, whole cloves, and bay leaf.
Cover pan entirely with aluminum foil. Place into oven and let it roast on 400° for the first 15 minutes.
After 15 minutes, turn down oven to 350° and cook lamb for 3 hours and 30 minutes until tender.
I usually like to check on the lamb as its cooking every hour to make sure it is looking right. You want the lamb to be very tender and juicy. Using a fork , shred a little piece on the side of the lamb to ensure it is cooking thoroughly. You want the meat to fall from its bone once its done.