Mongolian Beef is supposed to be just beef, scallions, and a little bit of seasoning. That's it. The seasoning depends on the chef. It started out as a lamb dish but became popular as beef because a lot of people didn't like lamb. I am no expert, but I do have 4 chinese chefs in my family. ;) My uncle used to cater Embassy dinners/parties in D.C, so I asked him.

The above is correct.
There is a great chinese place here in Pasadena-Fu Shing,they still have a lamb version as well as something called FireCracker lamb with hot chilis and scallion brushes.They also make the best pot stickers I have ever had!

My favorite style of Mongolian Beef is when it's in a thick sauce that is caramelized, and with green onions. At a local restaurant at college, they serve it with sliced white onions, which have not been cooked very long. Due to my general distaste of uncooked onions, I select only the meat, which is quite good.

That is how I prefer mines. ^ Makes me hungry now.