1. In a large soup pot over medium heat, heat oil and add celery root, celery, onion, garlic and leek. Sauté until vegetables begin to soften without browning, 10 to 15 minutes.
2. Add stock, potato, pears and bouquet garni. Bring to a boil, then reduce heat
and simmer until vegetables are tender, about 35 minutes. Season with salt and
pepper.
3. Strain soup through a coarse strainer, reserving vegetables and stock separately. Remove and discard bouquet garni. Using a blender or food processor, purée vegetables in batches, adding stock as necessary to blend smoothly. Strain soup through a fine- meshed strainer, and return to stove.
Adjust consistency: if soup is too thick, add a little stock. If too thin, simmer until thickened to taste. Adjust salt and pepper to taste, and if desired, swirl in butter to richen soup.
Yield: 6 to 8 servings.