For bouquet garni, place peppercorns, bay leaf, parsley, and thyme in center of an 8-inch square of cheesecloth. Bring up edges to form a bag; tie securely with kitchen string. Set aside.
In a large saucepan melt the butter over medium heat. Add leek. Cook and stir until softened, 6 to 8 minutes. Add garlic. Cook 1 minute more. Add water, celery root, salt, pepper and bouquet garni. Bring to boiling. Reduce heat. Simmer, uncovered, until the celery root fork tender, about 20 minutes. Remove from heat. Let cool 10 minutes.
Remove and discard bouquet garni. Working in batches, puree soup with an immersion blender or food processor. Return soup to low heat. Stir in cream. Heat through. (Do not allow it to boil.) Season to taste with salt and white pepper. Thin soup with chicken broth, if desired. (The soup can be prepared up to 3 days in advance. Let cool, transfer to a covered container, and refrigerate. Rewarm over low heat just before serving.)
To serve, top soup with creme fraiche, chives and a few fresh pear slices. Serve immediately.