1. Take 1 stick of celery and cut into thin strips, place in a bowl of iced water and set aside. Reserve a few leaves for garnish.
2. Reserve one quarter of an onion, then chop remaining onion and celery.
3. Melt butter in a saucepan, add onion and celery and cook for 5 minutes.
4. Stir in flour and cook for 1 minute, then gradually stir in milk. Add bay leaf, cover and simmer for 20 minutes.
5. Cool slightly, remove bay leaf, then puree soup in a blender or food processor.
6. Return to pan, stir in creme fraiche and add salt and pepper.
7. Reheat gently.
8. Chop reserved onion and add to soup. Drain celery curls and use to garnish soup with reserved leaves.