1. Pick over lentils to remove any stones, dirt, or other foreign objects. Rinse them well in cold water and place in a large pot with enough cold water to cover lentils by 6 inches. Add the bay leaf.
2. Bring to a boil, skim off foam, lower heat, and boil gently, partially covered, until lentils are just tooth-tender, 20-30 minutes.
3. Add carrots, celery, and onion to the lentils. Cook partially covered till carrots are tender, about 20-30 minutes.
4. Add crushed tomatoes, olive oil, and salt and pepper to taste. Simmer, partially covered, until lentils become very creamy and soft. Stir occasionally and add boiling water if necessary to prevent sticking.
5. Remove bay leaf before serving. If you like, stir in a little vinegar just before serving.
