Place celeriac and potato in a medium saucepan; add water to cover. Bring to a boil; cover, reduce heat, and simmer 20 to 25 minutes or until both are tender. Drain.
Pour celeriac and potato into a large bowl, and mash slightly with a potato masher. Add butter and remaining 3 ingredients; mash with potato masher until thoroughly blended. (Mixture will not be completely smooth.)
Celeriac, or celery root, is a rather unusual vegetable–knobby, hairy, and brown. Its intriguing texture combines the crunch of celery with the smoothness of potatoes. To prepare, simply trim the ends and peel away the rough exterior.
Oxmoor House Healthy Eating Collection