Fàntuán with Suāncài (饭团酸菜) is the glutinous rice roll built around pickled mustard greens, a sticky-rice cylinder whose anchor is sour rather than rich. The angle is acid against starch: the rice is heavy, chewy, and faintly sweet, and the suāncài is sharp, salty, and crunchy, so the roll works by cutting a dense, mild mass with a bright, fermented edge that keeps every bite from going flat. Get the balance right and it reads as rice lifted and sharpened by the pickle; get it wrong and the greens either drown in starch or release so much liquid that the roll turns to mush.
The build is short and turns on the rice and the drain. Glutinous rice is steamed until tender and tacky and spread warm onto a cloth or sheet. The suāncài, chopped and well squeezed of its brine, is laid down the center, and the rice is rolled tightly around it into a firm cylinder, pressed so it holds when handled. Good execution shows in the structure and the seasoning: the rice is warm enough to be pliant, the greens are drained hard so they crunch instead of weep, and the sourness is present in every bite rather than pooled in one spot. Sloppy versions fail plainly. Pickle packed in wet bleeds vinegar into the rice and the roll slumps into a sodden lump; too little suāncài leaves long stretches of plain starch and the whole point of the roll, that clean sour cut, never lands; rice rolled cold and stiff cracks and sheds. Some stalls over-sweeten the rice to chase a contrast and end up with a roll that fights itself, neither savory nor cleanly sour.
It shifts mostly by what joins the pickle inside. The leanest form is suāncài alone, sharp and spare. A length of yóutiáo, fried dough stick, is the usual addition, giving crunch and a touch of oil against the acid. Pork floss, ròusōng, is often layered in to add a sweet-savory richness that the sourness then cuts through, and that fuller build is really its own item driven by the floss rather than the pickle. Chili oil or a scatter of dried shrimp turns some versions hotter and more pungent. The all-savory egg-and-pickle roll is a close relative but reads as a different sandwich once egg leads, and the floss-forward version belongs in its own article rather than crowded in here.