How to Make It
In a medium saucepan, combine the celery root, potato, garlic, cream and milk and bring to a boil. Reduce the heat to moderately low and simmer until tender, about 40 minutes; drain in a colander set over a bowl and reserve the cooking liquid.
Working in batches, puree the celery root and potato in a blender, adding as much of the reserved cooking liquid as necessary to form a soft puree. Transfer the puree to the saucepan, season with salt and pepper and reheat before serving.