Place the potatoes in a large saucepan with enough water to cover. Bring to a boil, reduce to a simmer and cook until tender. At the same time, simmer the celery root in another saucepan until tender.
Gently warm the butter and cream in a saucepan over medium heat until the butter has melted. Drain the potatoes and celery root separately. Place the potatoes in a mixer and beat while carefully adding the warm cream/butter mixture.
Puree the celery root in a food processor and then beat into the potatoes. Season with salt and pepper and keep covered until service.
Recipe by Culinary Thymes at https://www.culinarythymes.com/instead-of-mashed-potatoes-try-potato-and-celery-root-puree/