Dry lamb: Remove lamb from the packaging and place it on a cutting board. Pat completely dry with paper towels.
Bring to room temperature: Let lamb sit at room temperature for 60 minutes. Meanwhile, trim and score the lamb, prep the pan and make the wet rub and spread it over the lamb anytime in the 60-minute window (directions below).
Trim and score: Trim exess fat (trim fat cap to about 1/8-inch). Score the lamb on both sides in a crosshatch/checkerboard pattern by running a sharp knife along the lamb in one direction, cuts about 1-inch apart, 1/4-1/2-inch deep, then the other direction.
Prepare pan and oven: Add the onions and garlic to the center of a large roasting pan, cut side up. Preheat oven to 325 degrees F. Season lamb: While the lamb is still on the cutting board, rub half of the wet rub all over the side of the lamb with the fat cap, making sure to massage into all the nooks and crannies from scoring. Don’t add the wet rub to the other side yet.
Add lamb to baking dish: Place the lamb, seasoned side down, in the pan so it’s propped up on the garlic and onions. Using your hands, rub the remaining wet rub all over the lamb.
Add broth and cover: Pour broth and water around the lamb. Cover with foil.
Slow roast: Bake at 325 degrees F for 3 hours. Check to see if the meat easily pulls from the bone with tongs after 3 hours. If not, continue to bake until tender.
Rest: Remove lamb and generoulsly spoon the pan juices over top. Transfer to serving platter and cover loosely with foil while you make the gravy.