Preheat oven to 400°F with rack in middle position. Place rutabaga, potatoes, celery root, cream, and thyme sprigs in a large saucepan. Cover and cook over medium, turning vegetables occasionally ...

Remove the pan from the stove, and put the potatoes through a ricer, or mash with a potato masher until they are smooth, or as smooth as you like them. Return them to the pot. 4.