How to Make It
In a large saucepan, melt the butter. Add the leek and garlic and cook over moderate heat, stirring occasionally, until softened, 5 minutes. Add the celery root, Parmesan chunk, stock and 5 cups of water and bring to a simmer. Cover and cook over moderately low heat, stirring occasionally, until the celery root is tender, 40 minutes.
Meanwhile, preheat the oven to 375°. Spread the walnuts in a pie plate and toast for 5 to 7 minutes, until golden. Let cool, then finely chop and transfer to a bowl. Add the oil, parsley and grated cheese and mix well.
In a blender, puree the soup in 2 batches until very smooth. Pour into a clean saucepan and stir in the heavy cream; season with salt and pepper and reheat if necessary. Serve topped with the walnut gremolata.