Preheat oven to 450 degrees.
Place the shanks in a roasting pan, just large enough to fit them in one layer and rub with olive oil. Season generously with salt and pepper. Roast for 20 minutes to get a nice brown color. Remove the lamb and turn the oven down to 325 degrees.
Spoon out any excess fat and scatter the garlic cloves and rosemary over and around the lamb. Wrap the pan very tightly with several layers of heavy-duty aluminum foil (you want as tight a seal as possible) and return to the oven.
Roast for 2 hours. Remove the foil, pour in the chicken broth and turn the heat up to 400 degrees. Roast for 15 minutes and remove. Transfer the lamb shanks to a serving platter and spoon over the juices, garlic cloves and rosemary.
Note: The soft garlic can be squeezed out of the skins and spread on the lamb.