Took me a bit to understand what was going on in those last few pages. What with the going back and forth on showing those two in their disguises and what they actually look like.
The buildup has been the whole manga.
Also, have you ever heard of superheated water? It can look calm outwardly, but disturb it a little and suddenly you have a steam explosion on your hands.
Also, this is just caramelized onions. As long as they have onions and can cook them, someone's going to find out about it just by accident. Why act as if they're unprecedented?