Potato, Chestnut, and Celery Root Purée

Preparation

1. In a large saucepan cook the onion in 1 1/2 tablespoons of the butter over moderately low heat, stirring, until it is softened, add the celery root, the potatoes, peeled and cut into 1-inch pieces, the chestnuts, the broth, and enough of the water to just cover the mixture, and simmer the mixture, covered, for 15 to 20 minutes, or until the vegetables are very tender. Drain the mixture, reserving the cooking liquid, force it through a food mill fitted with the medium disk or a ricer into a bowl, and stir in the remaining 1 1/2 tablespoons butter, salt and pepper to taste, and enough of the reserved cooking liquid to achieve the desired consistency. The purée may be made 1 day in advance, kept covered and chilled, and reheated. Transfer the purée to a serving dish and garnish it with the celery leaves.