celeriac
The thick, knobbly root bulb of the celery plant, with firm, creamy-white flesh and a strong celery flavour once peeled.
In German-speaking countries and the UK celeriac is in most supermarkets, while in North America it is usually found in well-stocked stores or at farmers' markets, often sold as celery root. The skin is thick and earthy and should be cut away generously.
Substitute:
Celery stalks give the flavour but not the starchy body; combined with a little potato or parsnip they come close in soups and stews.