In a large oven-safe pot, such as a Dutch oven, combine the lamb shanks, onion, garlic, bay leaves, thyme, and water. Bring to a boil, cover, and transfer to the oven.
Cook for 5 hours, or until the lamb is very tender.
Remove from the oven and let the lamb cool in the liquid until it is easy to handle.
Remove the lamb from the cooking liquid and set aside. Strain the cooking liquid into a small pot and discard the solids. Over high heat, reduce the liquid by half. Set aside to cool and skim and discard the fat that rises to the top.
While the cooking liquid is reducing, shred the lamb and discard the bones, fat, and membranes.
Season the lamb with salt and pepper to taste.