- 1 1/2 cups chopped onion (1 large)
- 3 garlic cloves, chopped
- 5 tablespoons unsalted butter
- 3 pounds 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 Turkish bay leaf or 1/2 California
- 1/2 teaspoon white pepper
- 6 cups water
- 3 cups reduced-sodium chicken broth
- 3 tablespoons extra-virgin olive oil
- 8 to 10 peeled roasted whole chestnuts (from a 7-ounce jar)
- 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. 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. Add water and broth and simmer, partially covered, until parsley root is very tender, 30 to 40 minutes.
- Discard thyme and bay leaf and stir in oil. Purée soup in batches in a blender until smooth (use caution when blending hot liquids), transferring to a bowl. If soup is too thick, thin to desired consistency with water. Season with salt, then return to cleaned pot to keep warm, covered, until ready to serve.
- Shave chestnuts with an adjustable-blade slicer or sharp vegetable peeler as thinly as possible over each serving.