1. Pick over the lentils and remove any stones. Rinse well.
2. Heat the butter or margarine in a pan and saute the onion for 2-3 minutes.
3. Add the diced celery and carrots and let the vegetables sweat for 5-10 minutes.
4. Stir in the lentils, add the lemon rind, stock and salt and pepper to taste.
5. Bring to the boil, reduce the heat and simmer for 15-20 minutes until the vegetables are tender.
6. Roughly blend the soup in a liquidiser; it should not be too smooth.
7. Check the seasoning and reheat gently.