For the lamb stuffing, season the lamb shoulder or shanks with salt, pepper, thyme, and pepper flakes. Heat the olive oil in a large heavy pot over high heat and sear the lamb on all sides until browned all over.

​Add the onions to the pot, reduce the heat to moderate, and cook, stirring occasionally, until the onions are brown, about 10 minutes. Add the garlic, tomatoes, rosemary, and Chicken Stock, increase the heat to medium-high and cook until the stock comes to a boil. Reduce the heat to low, cover, and cook until the meat is fork tender and falling off the bone, about 1½ hours.

​Transfer the meat from the broth to a platter (set the broth aside for later); season with salt and pepper. Set the meat aside and let cool to room temperature. 

​For the leg of lamb, preheat the oven to 400˚F. Lay out the leg of lamb, skin side down, on a cutting board and season with salt and pepper.

​Remove the cooled slow-cooked meat from the bones and arrange it in the center of the boneless leg of lamb (parallel to the short sides). Discard the bones. Beginning with a short side, roll the lamb around the slow-cooked meat, jelly-roll fashion, and tie it securely with kitchen string.

​Using a paring knife, make small incisions in the tied-up roast. Stuff a slice of garlic or a sprig of rosemary into each incision and season the roast with salt and pepper. Put the lamb into a roasting pan and roast until the internal temperature is 130˚F, 30-40 minutes. Remove from the oven and let rest 15 minutes.

While the lamb rests, reduce the broth of the slow-cooked meat by half over medium-high heat. Strain and serve it over slices of the stuffed leg of lamb.