2 large celery roots (about 2 1/2 pounds total), trimmed, peeled and cut into 2-inch cubes
1 medium russet potato (about 10 ounces), peeled and cut into 2-inch cubes
1 small onion, peeled and quartered
3 T. butter, cut into pieces
1 T. salt for the cooking water, plus more to taste
White pepper to taste
Milk as desired (optional)
1/2 tsp. freshly grated nutmeg

Fill a large pot of water, salt generously and add the celery root cubes, potato cubes, and onion quarters. Bring to boil. Reduce heat to medium and simmer until vegetables are tender, about 30 minutes. Drain and discard cooking liquid. Put vegetables and butter through a ricer or a food mill, or use a processor to puree until smooth. Stir in milk if you want it smoother. Season to taste with salt, white pepper and nutmeg.