Niúròu Jiāmó (牛肉夹馍) is the beef version of roujiamo, a split baked mó packed with slow-braised beef that has been chopped on the board with a little of its own jus and fat. The angle here is the braise as the whole event: where the famous pork roujiamo leans on rendered belly, the beef version trades that for shin and shank cooked long in a spiced master stock until the connective tissue turns silky, which makes it a common halal build in the Muslim quarters of Xi'an. Get it right and the mó holds a moist, deeply savory beef that almost shreds; get it wrong and the meat is dry and stringy or the bun goes soggy under too wet a chop.
The build has two halves and both have to land. The beef, usually a gelatin-rich cut like shank or brisket, is blanched, then simmered for hours in a stock dark with soy, dried chili, star anise, cassia, fennel, and clove, the pot often a perpetual lǔ base reused and replenished. When it is fork-tender it is lifted out, rested, and chopped on a block with a heavy knife, fat and lean cut through together, moistened with a spoon of the strained braise. The mó, a low-leavened wheat bun with a firm pale shell and a soft layered crumb, is baked or griddled until it has a crisp face, then split along its seam and the warm chopped beef pressed in. Good execution shows in the moisture and the structure: the bun stays crisp outside and the chop is juicy without running, the seasoning of the braise carrying through every bite, the beef tender enough to give but still in distinct pieces rather than paste. The failure modes are plain. Over-braised or over-chopped, the beef turns to mush and the bun to a wet sponge; under-cooked, it is tight and chewy with the connective tissue still rubbery; too dry a chop and the sandwich is tasteless flour and meat with no bind.
It shifts mostly by the cut, the spice of the braise, and what goes in alongside. Leaner cuts give a firmer, drier chop that needs more jus; fattier or more gelatinous ones run richer and glossier. Some stalls push the master stock heavy on chili and Sichuan pepper for a hotter, numbing read; others keep it close to a clean five-spice braise. Fresh chopped cilantro, pickled chili, or raw chili paste is often worked in to cut the depth. The pork-belly roujiamo is the better-known relative and runs on different logic, a fatty rendered braise rather than a lean spiced one, so it stands on its own. The cumin-lamb and the donkey-meat fillings are each their own preparation as well and deserve separate articles rather than being folded in here. What holds the beef version together is the long spiced braise chopped and pressed into a crisp split bun.