4large russet potatoespeeled and cut into 1 ½ inch chunks
2large celery rootspeeled and cut into 1 inch chunks
1tablespooncoarse saltplus more to taste
8tablespoons1 stick unsalted butter
Freshly ground pepper
Place potatoes, celery root and onion a large pot and cover with water and milk. Add salt. Bring to a boil over medium-high heat. Reduce heat to medium and continue to cook until potatoes and celery root are very tender, about 30 minutes. Drain and discard cooking liquid.
Transfer half of the potatoes and celery root to a food processor along with 4 tablespoons of butter. Process until smooth and transfer to a serving bowl. Repeat with remaining potatoes, celery root and butter. Season to taste with salt and pepper. Serve. (Puree can be prepared a day ahead and reheated in the microwave or on the stovetop just before serving).
This recipe can easily be halved for a smaller crowd.