We don’t often think to stir-fry potatoes, but that’s how they’re cooked in the classic Sichuan dish called tudou si. Cut into matchsticks, the potatoes are soaked to remove some of their starch, then are stir-fried until tender but with just a hint crispness at the core, not until totally yielding in texture. We flavor the potatoes with finely ground, tongue-tingling Sichuan peppercorns (pulverize them in an electric spice grinder), dried chilies for supplemental heat and Chinese black vinegar (also known as Chinkiang vinegar; balsamic vinegar is a reasonably good substitute) to add tanginess. The potatoes should be cut into ⅛-inch matchsticks, though the length of the sticks isn’t so important. We suggest using the julienne blade on a mandoline or, if you’re up for some knife work, do the prep by hand with a chef’s knife. Another option is to get a slicing assist from the mandoline or a food processor fitted with the ⅛-inch slicing disk, then cut the slices, stacked a few high, into matchsticks with a knife.