This is a tough question to answer, as the Mongolians, to the best
of my knowledge, don't make Mongolian beef. Back in the '60s and '70s
you used to get Mongolian lamb, which was meat stir-fried with onions
(at best, green onions), and not much else. I can't speak to the
authenticity of that dish, but it seems more plausible than most of
what we get in Chinese restaurants in the US these days.