Celeriac is a variety of celery selected for its enlarged root (actually a tuber). It is not common in America but is a stable in Europe. It has a more condensed celery flavor and celery is prepared like a root vegetable. Peel the skin of the celeriac bulb and cube. Then it can be roasted, stewed, or boiled and mashed. If you are looking for an easy way to prepare this unfamiliar vegetable try preparing mashed potatoes using half celeriac and half potatoes. It will store for a long time in the fridge in an airtight container.