I've heard that kashrut certification organizations require meat restaurants to have a mashgiach supervising operations at all times. What is the source for this requirement? (I find it unlikely that the early Halachic sources address the case of a restaurant under rabbinic supervision, as this is a fairly recent invention.)
Shulchan Aruch Yorah 63:1 tells us that the Rabbis decreed that any meat that has been out of eyesight of the Jew (even in his own house) is forbidden unless it has a sign on it, or the person can definitely recognize it. If it is wrapped and sealed it is not a problem.