Peel celery root with a sharp knife, then cut into 1-inch cubes. Peel and core apples, then cut into 1-inch pieces.
Melt butter in a 6- to 8-quart heavy pot over moderately low heat, then add celery root, apples, and salt and stir to coat with butter. Cover with a tight-fitting lid and cook (without adding liquid), stirring occasionally, until celery root is tender, 50 minutes to 1 hour.
Purée mixture in batches in a food processor until smooth, about 2 minutes per batch. Return purée to pot and stir in cream, white pepper, and nutmeg. Reheat, covered, over moderate heat, stirring occasionally, until hot, about 5 minutes.
Adapted from Gourmet, December 2005