Most people like to slice the beef first and then stir fry it, but have found it easier to cook it in the wok first and THEN slice it: it eliminates a lot of stirring while the meat cooks.
I like it best if I sprinkle a sliced onion with a LITTLE sugar and let it rest a few minutes. Then I stir fry the onion in a little oil, set the onion aside so that it does not overcook, then cook the meat in a wok (or a frying pan) and slice it.
THEN I combine the onion and the veggies in the wok with the sauce and heat briefly.
The sauce I use is simplicity itself: I use soy sauce and a little sugar.
Now, I will say that my sauce is not quite as good as the sauce of a professional. Even so, my stir-fry beef is excellent.
OH! Edited to add: the sauce can be modified by adding a little sliced ginger, or crystalized ginger, or Hunan sauce. I expect there are other modifications you can try also, but this is what I have tried. I still prefer the plain soy with just a little sugar, though.