Celeriac

This old-fashioned relative of celery also goes by the name celery root. The edible part is actually an enlarged knob of a stem that grows underground. Peel off the buff or tan outer layer to reveal the crunchy white edible interior.

You can eat it raw, but it's used most often to impart a mild celery flavor in soups, stews, casseroles, gratins, and other hearty baked dishes.