2 pounds Yukon gold potatoes, peeled and cut crosswise into 1/2-inch slices
2 pounds (1 large bulb) celeriac (celery root), peeled and cut in half, then cut into 1/2-inch slices
2 cups low-fat or whole milk
Kosher salt, to taste
4 tablespoons (1/2 stick) unsalted butter
White pepper, preferably freshly ground
Combine the potatoes, celeriac and milk in a large saucepan, then fill with enough cool water so liquid covers vegetables by about 2 inches. Bring to a boil over high heat; season generously with the salt. Reduce heat to low and cook for 20 to 25 minutes, until potatoes and celeriac are tender when pierced with a knife.
Drain vegetables in a colander, discarding liquid, and return them to saucepan over medium heat. Cook for 1 to 2 minutes, stirring constantly, until a floury film forms on bottom of saucepan.
Use a ricer, food mill or potato masher to mash vegetables in saucepan until smooth. Add the butter, stirring vigorously until well-combined. Taste and adjust seasoning with salt and the pepper as needed. Serve immediately or cool to room temperature, cover and refrigerate for up to 2 days. Reheat on stove top or in microwave. If consistency is too thick, add milk, butter or chicken broth to thin it.
Makes 4 to 6 servings.
— Recipe adapted by the Washington Post from "Bon Appetit, Y'all," by Virginia Willis (Ten Speed Press, 2008).