Celery root (also known as celeriac) is a roundish, knobby, brown-skinned root vegetable that tastes like a cross between parsley and celery. Despite its name, it's not the root of common stalk celery; it's cultivated from a special variety of celery.
Celery root is delicious both raw and cooked, imparting a subtle flavor to salads, soups and stews. Here we've teamed it with potatoes for side dish that's much more interesting than ordinary mashed spuds. For a richer, creamier version, substitute heavy (whipping) cream for the chicken stock.
Place the diced celery root and potatoes in a medium-size saucepan with the salt and water. Cover and cook over high heat until tender, 15 to 20 minutes.
Drain the vegetables and mash them with a potato masher or immersion blender. Return the purée to a saucepan and gently reheat over low heat.
Add enough chicken stock to moisten, and stir in the butter until melted. Season to taste with salt and pepper.