HIGH ON THE HILLSIDE in rural Mason County, shielded from the infrequent shafts of sunlight by translucent fabric, in gravel beds drowned by burbling artesian springs, a strange garden grows. Beneath the foot-high carpet of soft green stands a crowd of stout, knobbly stems with large, heart-shaped leaves spiraling around their girth. The plant being cultivated so carefully is wasabi, known to science as Wasabia japonica var. Matsu, and the artificial bed it's growing in has been contrived to match as closely as possible the conditions under which the plant grows in its native Japan: rooted among the rocks along the banks of narrow, cold, fast-flowing mountain streams seldom—if ever—touched directly by the sun. For most of us, "wasabi" is the little mound of bright green paste which comes to the table with your order of sushi, along with the pot of soy sauce and the paper-thin, salmon-colored slivers of ginger. But the odds are strong that, even if you've dined in Japan, you've never tasted fresh wasabi. The tangy, sinus-clearing stuff on your sushi-bar condiment plate is almost certainly made from reconstituted, dried powder—a powder composed mostly of plain old American-style horseradish at that, dyed green and zinged up with a trace of mustard and/or vinegar. Even in Japan, collecting the wasabi plant where it grows slowly in the wild is just too labor-intensive. A substantial portion of the fresh product (still available in its native land) originates in seven acres of hydroponic greenhouse near Florence, Oregon. North Americans too can sample the produce of Pacific Farms; but with a half-pound minimum shipment of a perishable condiment normally used by the gram costing nearly $37 via UPS, few but utterly dedicated gustatory adventurers have taken the plunge. Now Seattle gourmets can sample the exotic wasabi for as little as $3, and with no more effort than a visit to the north tables at the Pike Place Market, where Cathy Chadwick is spending her Wednesdays this spring introducing shoppers to the history, biology, and culinary uses of the unprepossessing root with the fiery flavor. Wasabi is a member of the wide-ranging botanical family of the crucifers, some cultivated for their fragrance (candytuft, stock), others for their leaves (cabbage, watercress), their roots (turnips, rutabaga), their edible flower buds (broccoli, cauliflower), even their seeds (mustard). Almost unique among its relations, wasabi is valued for its fibrous stem—its rhizome, to be botanically correct. The only thing the crucifers have in common, apart from the cross-shaped flowers which give them their scientific name, is a tendency to stock up on sulfur. It's sulfur compounds which give mustard seed, horseradish root, radishes, and cress (and overcooked cabbage) their pungency. It's sulfur compounds that make wasabi, when ground into a fibrous paste, a condiment to be reckoned with. THE TRADITION of serving wasabi with sushi may have medicinal as well as culinary roots. Modern chemists have rediscovered what Japanese folk-healers have always known: The aromatic sulfur compounds released when wasabi is finely ground are a powerful agent against the parasites sometimes found in raw fish. Once you've tasted it, however, you'll see why even people who wouldn't think of sullying their gullets with sushi still take to wasabi as a condiment. The flavor of just-ground wasabi is hard to describe: less hot than horseradish, it has a slight sweetness and appealing "green" taste which gives zest to its traditional partner without masking the delicate marine aroma of the fish itself. If sushi were the only food enhanced by wasabi, it would hardly be worth the home cook's attention. But as Chadwick has learned in 15 years of studying the plant, its useful range is far greater. Though only at its best served fresh, spoilage is not a problem: For all its finicky growing habits, wasabi is remarkably tolerant about how it's treated once harvested. Chadwick sells hers with a leaf or two still attached, to trick the plant into thinking it's still in one piece. Kept moist in a Ziploc bag in the fridge, wasabi will keep nicely for a month or more, ready for a portion to be ground to order the next time it's needed. "Grinding" sounds like a lot more trouble than it is. When you're ready to add a dollop of wasabi to a home-cooked dish, just strip the nubbly rind from a half-inch or so of the stalk with a vegetable peeler and rub the cut end against a porcelain ginger grater with a firm circular motion. (Cathy will show you how, and sell you the two-dollar grater if you don't own one.) At home, wasabi can replace the traditional dash of dry mustard in the making of a memorable shrimp salad or crab cocktail. When combined with sesame oil, scallions, and wine vinegar, wasabi turns a simple cucumber salad into a first-class starter course. You can blend freshly ground wasabi with a dab of butter melted into the pan juices of a saut饤 steak. (Don't cook it, though; if you do, the subtle warmth remains but the fragrance is lost.) With Chadwick's Olympic Mountain Wasabi just getting geared up for wide distribution, top-of-the-line sushi havens won't necessarily have fresh wasabi available yet (though Koji Matsumoto of Bainbridge Island's B.I. Sushi House, 108 Winslow Wy W, 780-9424, serves Chadwick's proudly). But why not do yourself a favor and simultaneously perform a public service? B.Y.O.W.: The sushi chef surely won't charge you corkage if you pull a porcelain grater and a stalk of wasabi from your purse, particularly if you offer him a bite of the result. Once he's tried it, you may never have to pull the same showy stunt again. Try the same trick at your favorite oyster bar. Purists may frown, but even they may be converted, once persuaded to try a dab of fresh wasabi with their next round of shooters. For more information or to order Olympic Mountain Wasabi, call 842-2119 or e-mail firstname.lastname@example.org.