For generations, Napa Valley has cultivated its image as carefully as its grapes: It is America's premiere wine region, known for its boutique operations turning out small quantities of some of the most sought-after wines in the world. But the utilitarian winery in Healdsburg where James Stewart produces his Slingshot Cabernet and Sauvignon Blanc throws a bucket of cold water on that romantic idea. It's a prefab warehouse in one of the valley's industrial parks, indistinguishable from countless other rent-a-fermenter facilities dotting California's wine belt. Amid rows of massive steel tanks and the woozy aroma of fermenting grape juice, Stewart is talking about turning the place into a gold mine.