Cōng (葱) is scallion, and it earns a page here not as a sandwich but as the single most load-bearing aromatic in the Chinese handheld repertoire, the green that turns plain dough and meat into something with lift. The angle is its dual role. Scallion is both a topping scattered on at the end for raw bite and color, and a filling worked into the dough or the meat so its flavor cooks through the whole thing. Understanding how a given sandwich uses it, raw and sharp on top versus softened and sweet inside, explains most of what you taste.
In practice scallion does its work in a few distinct ways across these breads and rolls. Sliced thin and raw, it goes onto a jiānbǐng or a bowl of filling as the last touch, contributing a clean allium snap that cuts fat and brightens everything under it. Chopped finer and folded into a dough with oil, it becomes the layered interior of a cōngyóubǐng, where pan-frying mellows it into a sweet, savory thread running through every flaky leaf. Cut into long batons, it is the cool, crisp counterpoint stacked alongside roast duck or braised pork in a folded wrapper, where its job is texture and a fresh edge against rich meat. Good use of scallion shows restraint and timing: enough that you taste green and onion, added at the point in cooking that suits the part it plays, with the white and green ends used for their different intensities. Poor use shows up as raw scallion dumped on in a coarse, bruised heap that overwhelms rather than lifts, or scallion cooked so long inside a bread that it goes grey and bitter and contributes nothing but a stringy texture.
It shifts mostly by which part of the plant is used and whether heat ever touches it. The thin green tops, raw, are the bright finishing note; the firmer whites, gently cooked, turn sweet and round. Some preparations infuse oil with scorched scallion and discard the solids, carrying only the perfume into a sauce. Related aromatics such as garlic chive, leek, and the small spring onion play near-identical roles in their own preparations and deserve their own treatment rather than being folded in here. What keeps cōng its own entry is how often a Chinese sandwich is built specifically to frame it, the bread or the meat acting as the steady base and the scallion supplying the part that makes the bite feel alive.