From the Urban Kitchen website (http://www.urbankitchentulsa.com/)
What in the world is celeriac?
Celeriac is a really ugly, knobby brown vegetable that is also known as celery root or celery knob...and it is delicious!!
It is NOT the root of our common celery but is a special celery cultivated solely for its root.
Celeriac (moving forward I will refer to it as celery root) tastes like a milder version of celery although some say it tastes like a cross between celery and parsley but I don't get the parsley flavor, at least not yet. It can be used in a variety of preparations from mashed (like potatoes) to roasted to raw. The cooked flavor is buttery and rich with a hint of celery flavor. Raw, it is a bit stronger but very nice shredded into salads or cole slaws.
When preparing to use celery root, cut off the top and bottom so that you have a solid foundation, then start cutting the outer skin off from top to bottom as you might a pineapple until you've worked all the way around it. It still looks really ugly at this point but, trust me, you are ready for a treat. After you have diced, shredded or otherwise "dismembered" the root, place it in a lemon water bath (or acidulated water) so that it won't turn brown...kind of like potatoes.
Celery root is available October through April and can range in size from apple to cantaloupe. I find that the larger ones (although impressive) tend to have a pithy spot in the center. It doesn't seem to affect the taste or texture so it probably doesn't really matter...just thought I would let you know.
So...start experimenting! It is a great alternative to mashed potatoes and is delicious added to roasted meats. It can be boiled, braised, sauteed or baked...or try it raw in your next cole slaw or salad...just use your imagination!