Preparation Place celery root and potatoes in large saucepan. Cover with water; salt lightly. Boil until very tender, about 15 minutes. Drain vegetables, reserving 1/4 cup liquid. Return vegetables to same saucepan. Add cream and butter and mash until almost smooth. Season with celery salt and pepper. (Can be prepared 1 day ahead. Cover puree and cooking liquid separately and refrigerate. Stir puree over medium heat until heated through, adding reserved cooking liquid if mixture is dry.)