Potato and Celeriac Mash

Ingredients

• 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

Directions

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.