In a large saucepan, cover the beans and the bay leaf with 2 inches of water and bring to a boil. Simmer over low heat until the beans are tender, about 2 hours. Drain the beans and discard the bay leaf.

Meanwhile, preheat the oven to 300 degrees. Heat an enameled cast-iron casserole that's large enough to hold the lamb shanks in a single layer. Season the shanks with salt and black pepper and cook over moderate heat, turning a few times, until lightly browned all over, about 15 minutes. Nestle the garlic cloves among the shanks. Cover and cook in the oven for about 1 hour and 45 minutes, turning 3 times, until the shanks are very tender.

Reduce the oven temperature to 200 degrees. Transfer the shanks to a small roasting pan, and the garlic cloves to a small bowl. Cover the shanks with foil and keep warm in the oven. Strain the juices from the casserole into a bowl and skim off the fat. Return the juices to the casserole, add the chicken stock and set the casserole over a burner. Boil over high heat until the juices have reduced to 2 cups, about 12 minutes.

Peel the garlic cloves and add the sherry vinegar. With a fork, mash to a paste. Add the garlic paste to the juices in the casserole and stir in the beans and thyme. Simmer over low heat for 5 minutes. Season with salt and black pepper.

Place the lamb shanks on plates and serve with the beans.

The beans can be prepared through Step 1 and refrigerated overnight in their liquid. The lamb can be prepared through Step 2 and refrigerated overnight.