1. Heat the butter and gently fry the onion and celery, covered, over a low heat until soft but not coloured.
2. Meanwhile, peel the carrots. Chop two of them and grate the third. Peel and chop the Jerusalem artichokes. Add the chopped carrots and artichokes to the onion and celery and fry gently another minute or so.
3. Pour over the stock or water, bring to the boil, then reduce the heat and simmer gently, partially covered, until the vegetables are soft, about 1520 minutes.
4. Allow to cool slightly before puréeing in a blender or food processor, taking care if the mixture is too hot. Return the puréed soup to a clean saucepan, taste for seasoning and add salt and pepper. Heat gently and stir in the single cream for extra richness, if using. Do not boil after adding the cream.
5. To serve, ladle into bowls and top with the reserved grated carrot to give a nice colour contrast. Serve with crusty French bread or croutons.