Bringing in the flavor of celery root with the lightness of mashed potato, this purée is a simple, unassuming side dish. But, it tends to "wow" people who have never experienced celery root before. Serve it in place of mashed potatoes, or on its own with sautéed greens.
Wash the celery root, then peel it with a sharp vegetable peeler or a knife to remove all of outer layers until there is only the smooth cream-colored flesh. Slice the celery root into slices 1 inch thick and then into cubes.
Put the celery root and the potato into a sauce pan. Add the stock, 1 teaspoon of the salt, garlic and bay leaf. Bring the liquid to a boil and then simmer until the celery root and the potato are fork tender, 25 to 30 minutes.
Strain the vegetables well (reserve this liquid for soup). Reserve the garlic, but discard the bay leaf. Put the celery root, potatoes, garlic, butter, sour cream, if using, the remaining 1/4 teaspoon salt and pepper into the bowl of a food processor.
Puree until the mixture is very smooth. Taste for seasoning and either serve immediately or keep warm by transferring the puree into a metal bowl and setting it over a pot of simmering water with foil or parchment paper pressed down onto the surface to prevent drying.
Leftover celery root purée becomes a lovely soup--one of the most popular uses for this root vegetable in European cuisines. Just add the reserved broth, milk, cream or a combination to get the consistency you like and adjust the seasoning with salt and pepper.