1 large onion
3 garlic cloves, chopped
5 tablespoons unsalted butter
1.5kg parsley root (about 4 1/2 pounds total with tops), tops discarded and root peeled and chopped
3 (4- to 5-inch) sprigs thyme
1 bay leaf
1/2 teaspoon white pepper
125ml chicken stock
3 tablespoons extra-virgin olive oil
8 to 10 peeled roasted whole chestnuts
1. Cook onion and garlic in butter in a large heavy pot over medium heat, stirring occasionally, until onion is softened and golden, 6 to 8 minutes.
2. Add parsley root, thyme, bay leaf, white pepper, and 3/4 teaspoon salt and cook, stirring occasionally, until parsley root begins to soften, 8 to 10 minutes.
3. Add water and broth and simmer, partially covered, until parsley root is very tender, 30 to 40 minutes.
4. Discard thyme and bay leaf and stir in oil.
5. Purée soup in batches in a blender until smooth, transferring to a bowl. If soup is too thick, thin to desired consistency with water.
6. Season with salt, then return to cleaned pot to keep warm, covered, until ready to serve.
7. Shave chestnuts with an adjustable-blade slicer or sharp vegetable peeler as thinly as possible over each serving.