Celery root (also called celeriac) adds a mild earthy flavor to this creamy side dish. Thin the puree with some warm milk or low-sodium broth for a silky, satisfying soup.
In a large pot, bring potatoes and celery root to a boil in salted water. Reduce to a rapid simmer and cook until vegetables are tender when pierced with a knife, about 15 minutes; drain and transfer vegetables to a food processor. Add milk and butter. Puree until smooth, scraping down side as needed. Season with salt and pepper and transfer to a serving dish. Serve warm.