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.)