Put a large pot of water on high heat. Add the celery root and salt and bring to a boil. When the water has been boiling for 10 minutes, add the potatoes. Cook just until a skewer can easily penetrate the center of the potatoes and the celery root, 30 to 35 minutes total.
Draw off about 1 cup of the cooking liquid and set it aside. Drain the potatoes and celery root and pass them through the fine disk of a food mill back into the pot in which they were cooked. Mix in a couple of tablespoons of the cooking liquid to loosen the mixture. Using a wooden spoon, mix in the butter. Bring the mashed potatoes to the consistency you like by adding a little more cooking water, if necessary. Season with salt and pepper and serve.
nutrition information (per serving):
Photo: Ellen Silverman