Preparation Peel potatoes and celery root. Cut into 1-inch chunks. Steam in a large steamer set over boiling water, covered, until very tender, 30 to 40 minutes. Force through food mill into a bowl. Stir in cream, chervil, 3/4 teaspoon salt, and 1/2 teaspoon pepper. Season with additional salt. Cooks' note:Purée can be made 2 days ahead and chilled. Reheat gently.
Preparation Peel potatoes and celery root. Cut into 1-inch chunks. Steam in a large steamer set over boiling water, covered, until very tender, 30 to 40 minutes. Force through food mill into a bowl. Stir in cream, chervil, 3/4 teaspoon salt, and 1/2 teaspoon pepper. Season with additional salt. Cooks' note:Purée can be made 2 days ahead and chilled. Reheat gently.