1. Boil: Fill a large pot with salted water. Place over medium-high heat and bring to a rolling boil. Add celeriac and potatoes. Return to a boil and simmer for about 20–25 minutes, or until celeriac is tender and easily pierced with a fork.2. Puree: Remove from heat. Set a food mill fitted with a medium blade over a large bowl. Using a slotted spoon, remove celeriac and potatoes (reserve some of the cooking water), and transfer to the food mill. Rotate food mill until all of the celeriac and potatoes are pureed; you may need to do this in batches. Add butter or olive oil to the hot milled mixture; stir to dissolve and blend. (Alternatively, you can use a food processor fitted with an “S” blade. Process celeriac and potatoes with butter or oil by pulsing until pureed.)3. Adjust and season: Add a little of the reserved cooking water to mixture only as needed to thin consistency if too thick. Season to taste with plenty of salt and pepper and herbs if desired.Roasted Garlic Variation: For a stronger flavor, add 2 roasted garlic cloves (peels removed) to the food mill prior to pureeing.Serves: 6–8