Melt butter in heavy large pot over medium heat. Add celery; cover and cook until slightly softened, about 3 minutes. Add shallots; sauté uncovered 3 minutes. Stir in celery root cubes and potato, then broth and 1 1/2 teaspoons thyme. Increase heat to high; bring to boil. Reduce heat to medium-low, cover, and simmer until vegetables are very tender, about 40 minutes. Cool slightly.
Working in batches, transfer soup to blender and puree until smooth. (Can be prepared 2 days ahead. Cool slightly. Cover and refrigerate.).
Stir cream into soup and bring to simmer. Season to taste with salt and pepper. Ladle soup into bowls. Sprinkle with additional chopped thyme and serve.