Preparation 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.
Preparation 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.