Heat a large heavy based saucepan over a moderately high heat. Seal the lamb shanks in the oil for 8 minutes or until well coloured, then set them aside.
Add the onion, carrot and celery and cook for 3 minutes. Add the spices and stir for 30 seconds. Place the shanks back in and the add tomato puree and stock. Bring to the boil, cover and turn down to a gentle simmer. Cook for 2 hours or until the meat is very tender. Take the pan off the stove.
Remove the shanks and set aside until cool enough to handle. Pull the meat off the bone and shred it with forks, discarding any fat or sinew.
Set the soup over a moderate heat and add the shredded meat and cous cous. Cook for 10-15 minutes or until the cous cous is cooked. Season with salt and pepper. Add lemon juice to taste, sprinkle with chopped parsley and serve.