Cooling cooked onions before adding to raw mince
I don't buy his reasoning, though I don't know for sure. So, if someone knows better, I am happy to be corrected. However, I do have two theories:  1.  I suppose it is possible that the heat from the onion will start melting the fat in the ground meat...but would that keep it from holding together?  I doubt it. Plus, that looked like a fairly lean mixture.   2.  I could see cooling the onion as a safety precaution, in the event the burger wasn't going to be cooked right away.  I think this is a much more plausible reason to cool burger additions.