Chestnut and Celery Soup

Preparation

1. In a kettle cook the onion, the garlic, the bay leaf, and the thyme in the butter over moderately low heat, stirring, until the onion is softened, add the celery and the carrots, and cook the mixture over moderate heat, stirring, for 5 minutes. Add the chestnuts, the potato, peeled and cut into 1-inch pieces, the broth, 2 1/2 cups water, and salt and pepper to taste, simmer the mixture, covered, for 20 to 25 minutes, or until the chestnuts and the potato are very tender, and discard the bay leaf. In a blender puree the mixture in batches until it is smooth and return it to the kettle. The soup may be prepared 1 day in advance, cooled uncovered, and kept covered and chilled. Heat the soup, stirring, until it is hot and serve it with the sour cream.