- 2 1/2 pounds German Butterball, Yellow Finn or Yukon Gold potatoes
- 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.