Preheat the oven to 275 degrees F. Place the lamb on a cutting board and make a small slit in the top using the tip of a paring knife. Insert a garlic clove into the slit, pushing it down so it is completely tucked in to prevent it from burning while the meat cooks. Repeat with the remaining garlic cloves.
Place the potatoes, onions and thyme leaves in a large bowl. Season with salt and pepper and toss to combine. Shingle the potatoes and onions in the bottom of a roasting pan, overlapping them slightly so you fill the pan with an even layer. Set the lamb on top, skin-side up. Pour the chicken stock over the lamb and roast until the potatoes are crisp on top and soft inside, 4 to 5 hours.
Remove the lamb from the roasting pan and set it on a cutting board. Loosely tent with foil and set it aside to rest for 20 minutes before slicing into pieces. Serve the lamb, onions, and potatoes with French beans (or any green vegetable of your choice).