Set up your grill for indirect heat. You want to target 325F and stay there if you can. If there was ever a recipe that favours a kamodo, this is it. If you are using a different style grill, plan to add charcoal along the way.
Remove the leaves from 2 sprigs of rosemary. Tuck them into the flap where the bone was along with the garlic slices.
Season the lamb generously with coarse ground pepper. You don''t need any more salt.
Place the lamb in a roasting pan and roast it until it reaches an internal temperature of 130F, about 70-90 minutes. Spin the roasting pan every 20-30 minutes to manage any hot spots on your grill.
Remove the lamb from the pan and let rest, covered, for about 20 minutes.
Strain the pan juices and spoon off any obvious fat. Return the juices to the pan along with the lemon juice. Bring to a simmer.
Carve the lamb across the grain. Pour any accumulated board juices back into the pan with the lemon sauce.
Serve the lamb with a drizzle of sauce.