ISO: Restaurant style beef and broccoli ?

I have been trying to find out how to recreate restaurant style stirfry beef
and broccoli. I would love to know what the sauce is made of and how to make
it at home. Some say it's just soy sauce that is thickened with cornstarch,
some say it's oyster sauce, some say it's soy sauce with pineapple juice
added. I'm wondering if it is a sauce that some restaurants buy in bulk
containers, as so many restaurant's beef and broccoli sauce tastes the same.
Does anyone actually know what the majority of chinese restaurants use for this
rich brown sauce that they use with beef and broccoli?

Any help would be very much appreciated!