Put the lentils into a large pan with the onion, celery, bay leaf and stock or water. Season with pepper and cumin, bring slowly to the boil, then cover and simmer until the lentils are soft, about 40 minutes. Remove from the heat and purée until smooth. Add the lemon juice.
While the lentils are cooking, scrub the mussels and remove the beards, discarding any that are broken or