Melt the butter in a large saucepan. Add the onion, leek and garlic and cook on a gentle heat for 6 to 8 minutes, until softened. Add the celeriac, potato and bay leaf and cook for another minute or two. Season the mixture lightly with salt and pepper at this stage.
Add the stock, cream and sage. Bring to the boil and allow to boil for 2 to 3 minutes, then reduce the heat to a gentle simmer. Simmer for an additional 10 to 15 minutes, until the vegetables have softened.
Stir in half the blue cheese, then blend the soup with a hand-held blender or in a food processor, adding additional stock if necessary until it’s the consistency of thick cream. Taste and adjust the seasoning if required.
To serve, ladle the soup into warmed bowls, then crumble in the remaining blue cheese and garnish with a fresh sage leaf on top.