Chop the celery, onion and walnuts. Cook the vegetables in the butter or margarine until softened. Reduce the heat, stir in the walnuts and flour.

Gradually add the stock, stirring all the time. Bring to the boil, reduce the heat and simmer gently for 20 minutes.

Season the soup, then transfer it to a liquidiser and blend until smooth. Return the soup to a saucepan and reheat over a low heat. Crumble the Stilton and add it to the soup, then stir in the port, if using.

Pour the soup into individual soup bowls and garnish with celery curls and leaves or croutons.