How to Make It
Preheat the oven to 300°. Season the lamb shanks with 1 teaspoon of the salt and 1/2 teaspoon of the pepper.
In an enameled cast-iron casserole just large enough to hold the lamb shanks, combine the shanks with the unpeeled garlic cloves, bay leaves and thyme sprigs. Cook the shanks over moderate heat until evenly browned, about 15 minutes. Cover the casserole and roast the shanks for 2 hours, or until very tender, turning every 20 minutes. Remove from the oven and let stand, covered, for 20 minutes.
Remove the shanks and set them aside. Add the chicken stock to the casserole and bring to a boil over high heat, scraping up any browned bits from the bottom. Boil, skimming frequently, until reduced to 2 cups, about 10 minutes.
Strain the stock and return it to the casserole. Add the peeled garlic cloves and simmer gently until tender, about 20 minutes. Return the lamb to the casserole and add the bell peppers and thyme leaves. Cover and cook until the peppers are tender, about 10 minutes.
Using a slotted spoon, transfer the lamb shanks to warm serving plates. Stir the butter into the sauce and season with the remaining 1/4 teaspoon of salt and 1/2 teaspoon of pepper. Spoon the sauce around the shanks and serve.