Heat oil in a casserole over medium-high heat until the oil begins to smoke. Brown the lamb shanks in the hot oil on all sides until well browned, about 10 minutes. Remove lamb shanks and set aside. Pour the excess grease from the casserole, reduce heat to medium, and stir in the onions and garlic. Cook and stir until the onions have softened and turned translucent, about 5 minutes. Stir in the carrots, celery and tomato puree; continue cooking 5 minutes more.
Return the lamb shanks to the casserole, and pour in the Guinness® and beef stock. Bring to a simmer over high heat. While you're waiting for the Guinness® to simmer, use kitchen string to tie together the thyme sprigs, parsley sprigs and bay leaf into a secure bundle; add to the lamb shanks.
Once the lamb shanks begin to simmer, reduce the heat to medium-low, cover, and simmer until the lamb is very tender and nearly falling off of the bone, 2 to 3 hours. Stir the lamb occasionally as it cooks, and add water if needed to keep the cooking liquid from becoming too thick. You want the cooking liquid to have reduced into a nice sauce by the time the lamb shanks are done. Stir in the rosemary sprig, and salt and pepper to taste during the last 10 minutes of cooking. Remove rosemary sprig and herb bundle before serving.
This is excellent. My partner is Hindu, so I used lamb stock (cube) instead of beef. I've got rosemary in the garden, but not the other herbs listed, so I used Bart dried 'Mediterranean' mixture. I served it with mashed potato and steamed broccoli. Just right for an autumn day! - 09 Nov 2015
This was very good indeed. Didn't tweak anything but the amount because there are only 2 of us. Next time will make 2 smaller casseroles and freeze one for another time. Thanks for sharing. - 13 May 2014