Season lamb shanks liberally with salt and pepper.Heat the olive oil in a heavy bottomed pot or Dutch oven over medium-high heat. Brown the shanks on all sides in batches, about 10 minutes per batch. Remove shanks and set aside.Pour off all but 1-2 tablespoon of the remaining oil and drippings. Add the shallots and mushrooms to the pot and cook until just starting to brown, about 5 minutes. Add the garlic, carrots, celery and tomato paste and cook 5 more minutes, stirring occasionally. Pour in the stout beer and scrape up all of the brown bits in the bottom of the pot.Return the shanks to the pot and pour in the beef stock. Turn the heat to high and bring the liquid to a simmer. Tie the herb sprigs into a bundle with twine and add them to the pot.When the liquid comes to a heavy simmer, lower the heat to medium-low and cover. Cook for 2½ hours, turning the shanks halfway through, or until the meat is almost falling off the bone. Remove the shanks and set aside.Increase the heat to medium-high and bring the sauce to aboil. Remove the herb bundle and discard. Allow to simmerand reduce to preferred consistency. Serve the shanks withsauce.