This fluffy bowl piles on the roots, with celery root and horseradish joining the mix. Celery root, also known as celeriac, is the dark horse of root vegetables. Combine the potatoes and celery root in large pot and cover with cold water. Bring to a boil and simmer, partially covered, until the potatoes and celery root are very tender, about 20 minutes. Stir in the sour cream, 2 tablespoons cheese, 2 tablespoons horseradish and the pepper.