At the Farmers' Market: Celtuce (Wo Sun)
Besides the huge variety in brassicas (there are a couple of dozen varieties of bok choy alone), there are some speciality vegetables that until recently were simply unavailable to the general public.
Even Fuchsia Dunlop, in her Sichuan cookbook Land of Plenty, cautions makers of fish-fragrant pork slivers (yu xiang rou si, 魚香肉絲) that they'll need to substitute for the lettuce stems called for in the original Sichuan recipe.
Fortunately, thanks to the crowded tables at our farmers' markets, that is no longer true: Lettuce stems, also called celtuce or wo sun (莴笋), are available and currently in season. They're exactly what they sound like: tall stalks that resemble extremely thick, pale asparagus that are topped with a sheaf of long leaves.
Like asparagus, look for perky tops and freshly cut bottoms (the browner the bottom, the longer the plant has gone since harvest). A quick look at the cut will show the color change between the outer and inner part of the plant; the dark ring is tough and fibrous, and the paler center is tender. Rather than waste half the plant by snapping it, peel the fibrous section off with a vegetable peeler. The leaves are perfectly edible and can be stir-fried as any leaf.
The vegetable, cut on the bias, has the texture of celery but a less alkaline and more grassy flavor. It tastes good raw (and makes excellent slaw) and makes surprisingly good pickles, but its spiritual home is in the aforementioned dish of slivered pork stir-fried with cloud-ear mushrooms, wo sun, garlic, ginger, pickled chile paste (sambal oelek), Shaoxing rice wine, soy sauce, sugar and black Chinkiang vinegar.