Given the background description of some cooking related query, summarize the question into a title

[EX Q]: I understand that food cooks differently in a double-boiler than when cooked "directly" - but don't know why that is.How does utilizing a double-boiler change how food cooks?
[EX A]: Why does food cook differently in a double-boiler?

[EX Q]: I am a cooking dummy and I hate bitter cucumbers. :) Help!
[EX A]: How to prevent cucumbers from tasting bitter?

[EX Q]: In Masterchef The Professionals (season 8 episode 11), Marcus Wareing demonstrates his skills test and cooks a burger. In the process, he rapidly cools the onions after sweating them by placing them on a freezing tray. Would it make a noticeable difference to skip that step? I never did this and my burgers did not fall apart as he suggests. Is there some other reason for doing this?You can see the episode here (skip to 17:30).
[EX A]:
Cooling cooked onions before adding to raw mince