Tǔdòu Sī Jiāmó (土豆丝夹馍) is the shredded-potato roujiamo, a split baked mó packed with stir-fried julienned potato seasoned with vinegar and chili, a vegetarian street build that runs the same bread as the braised-meat version with a sharp, crunchy filling instead. The angle is texture against a soft bun. Where meat roujiamo trades on rich braise soaking into the crumb, this one is about a tangle of barely-cooked potato that has to stay crisp and tart inside a warm pocket, so the whole thing hinges on the potato keeping its snap rather than steaming to mush against the bread.
The build is the standard mó filled with a quick stir-fry. Potatoes are peeled and cut into fine even matchsticks, then rinsed hard to wash off surface starch so they fry up separate and crunchy rather than gluey. They hit a very hot wok briefly with oil, dried chili or chili oil, and a hit of vinegar added near the end so the acid stays bright and the potato keeps a raw-edged bite, often with a little salt, scallion, or Sichuan pepper. The baked mó is split most of the way through and the hot potato is packed in, sometimes with a spoon of the chili oil from the pan. Done well it shows a mó with a firm, lightly crisp shell and a soft inside, a filling that is still crunchy and audibly so, a clean tartness from the vinegar, and chili heat that carries without burying the potato. Done poorly the failure modes are immediate: potato cut thick or under-rinsed so it turns soft and starchy and pastes together, cooked too long so it goes limp and the vinegar boils off flat, too much oil so the bun goes greasy at the seam, or so much filling that the mó splits and the potato spills.
It shifts mostly by the heat and acid level and by what joins the potato. Some stalls run it fierce with dried chili and Sichuan pepper for a numbing, sour-hot profile; others keep it mild and lean on the vinegar alone. Green pepper, carrot, or cilantro sometimes go in for color and bite, and the chili can be fresh, dried, or a spoon of oil. It belongs to the wider roujiamo family that shares the split-mó format, where the braised pork, beef, and cumin lamb versions are each their own preparations rather than crowded in here. What fixes the potato version as its own entry is the vinegar-and-chili stir-fried julienne, kept crisp and tart, standing in for meat as the entire filling.