Cut the garlic and onion into thin slices. Chop the celery and carrots.
In a heavy-bottomed, lidded pan heat 1 Tbsp oil, brown the lamb all over and set aside.
Add the remaining oil to the pan and sauté the chopped vegetable for about 10 minutes until soft. Return the shanks, add the beer and stock, then the bay leaf and thyme. Season with salt and pepper.
Bring it to the boil and simmer, under cover, at a low temperature for 2- 2 and a half hours. Turn over the shanks a couple of times during cooking.
When the meat is tender and pulls off the bone, remove the lamb to a plate and cover with foil.
Bring the sauce back to the boil and reduce it to a thick, silky gravy. Stir in the parsley.
Serve the lamb shanks with the gravy poured over them.