First, peel and finely chop the celeriac, but keep the peel. Pour the chicken stock into a large saucepan over a high heat and bring to the boil. Add the celeriac peel, then remove the pan from the heat, cover the top with clingfilm and leave to infuse for 20-30 minutes.
Heat the butter in a large saucepan over a low heat. Add the onion and a pinch of salt, cover the pan and leave the onion to sweat for 10-12 minutes until softened, but not coloured.
Meanwhile, peel and dice the apples When the onion has softened, add the apples and any juice, and the celeriac. Strain the infused chicken stock into the pan and bring to the boil. Reduce the heat to low and simmer, for 10-15 minutes until the celeriac is tender. Add the double cream and return the liquid to the boil. Pour the soup into a blender and blend until smooth.
When ready to serve, reheat the soup, if necessary. Crumble the blue cheese into soup (if using) bowls, then pour the hot soup on top and finish with toasted walnuts. If liked!