The short version
Mongolian beef is flank or similar steak, sliced thin, velvety with cornstarch, then glazed in a sweet-savory soy sauce. Broccoli often tags along in American kitchens because green makes the plate look like dinner. Mongolia is not involved. There are no steppes in this wok.
Where the name wandered in
“Mongolian barbecue” as a restaurant format is usually traced to Taiwan in the mid-20th century — a Japanese-influenced grill-your-own show, popularized by showman Wu Zhaonan, not by Mongolian herders. American Chinese menus borrowed the word because it sounded bold and a little exotic, then applied it to a stir-fry that is closer to brown-sauce beef than anything from Ulaanbaatar.
Chains, especially from the 1990s onward, locked in the sweet glaze and the onion-heavy sauté that most takeout boxes now copy.
Legend vs ledger: If someone tells you Genghis Khan tossed flank steak in brown sugar, smile politely and pass the rice. The dish is American Chinese (with a Taiwanese-adjacent nickname), full stop.
Why the American version tastes like that
Velveting — cornstarch and a brief marinade — is the Chinese-American restaurant trick for tender slices in a hot pan. Brown sugar and soy give the lacquer. Broccoli is the weeknight vegetable that survives a glaze. Our version stays under 30 minutes and keeps the MSG out; the bouillon and soy do the savory work.
Cook it
Flank steak, broccoli, sweet-savory glaze, table in a half hour.