[Celeriac, Knob celery, Turnip rooted celery, Apium graveolens var rapaceum]
Developed in Europe during the Renaissance, this form is particularly popular in France, Germany, Poland, Russia and Turkey. Once little known in North America, celeriac has lately become more widely available, with California a major producer. The part eaten is not actually a root, but a swollen stem, from which the leaves sprout. These "roots" range in size from 7 ounces to over 2 pounds. The larger of the photo specimens was 6-1/2 inches long, 4-1/4 inches diameter and weighed 2 pounds.
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A strongly aromatic vegetable, celery root is useful in soups, stews, mashed potatoes, and various cooked and uncooked salads. The leafy tops, if included and in good condition, are an excellent flavoring when making soup stock, much stronger than regular Pascal celery.
Buying: While many produce markets and supermarkets here in Southern California stock these, they may be less available in regions that do not have large ethnic populations. Look for very firm roots with stems and leaves that are still green and unwilted. Buy medium size roots with many stems, as very large ones, and especially ones with just one thick stem coming out of them, tend to be very fibrous and totally unfit for salads.
Storing: These will keep for weeks, even a month or more, in the crisper drawer of your refrigerator. Wrap loosely in plastic and make sure there's air circulation so they can breathe.
Prep: Celery roots are rather hard and daunting in appearance, but they peel easily with a swivel vegetable peeler. In the grungier areas it'll take several strokes of the peeler to clean them up, and possibly a little work with a sharp pointed knife. An easy and safe way to start cutting a large one is to drive a sharp Chinese cleaver knife through it lengthwise, with a soft faced mallet. Roots to be used raw are often grated on the large hole side of a box grater, or they can be cut into shavings with your swivel peeler, or narrow strips with a julienning peeler.
Cooking: These "roots" are often cooked whole and unpeeled in boiling water to preserve their flavor. Take care not to overcook them, they cook faster than you might think, and are done rather suddenly. Check frequently with a sharp skewer - as soon as it doesn't encounter a hard center they're done.
When done, they are often cut into narrow juliennes. They are too soft to use julienning tools, so just slice them 1/8 inch thick, then skew the slices on your cutting board so they overlap and cut crosswise into strips - very easy.