Source: Bon Appetit, December 2011

    • 2 1/2 pounds mixed russet, Yukon Gold, and white-skinned potatoes, peeled, cut into 2″ cubes
    • 1 1-pound celery root (celeriac), peeled, cut into 3/4″ cubes
    • 1 6″ piece of horseradish, peeled, coarsely grated
    • 1 1/2 cups sour cream
    • 3 tablespoons Dijon mustard
    • 2 tablespoons (1/4 stick) unsalted butter
    • Kosher salt
      • Place potatoes, celery root, and horseradish in a large pot. Add water to cover by 1″. Bring to a boil, reduce heat to medium-high, and simmer until vegetables are tender, 25-30 minutes.
      Drain, reserving 1 cup cooking liquid. Return vegetables to pot; add sour cream, Dijon mustard, and butter. Using a potato masher, coarsely mash vegetables. Add reserved cooking liquid by tablespoonfuls if mash is too stiff. Season to taste with salt.