I have no issues with red onions, but then again I train (strengthtrain) and move a lot as being ketogenic makes me have so much energy that I get jittery if I stay sedentary the whole day 
Darn keto for making me full of energy all day long and making me lead an active life 
That’s the theory. Whenever this is actually tested in the lab, it doesn’t actually show this much, if at all. It’s just a tale folk like Dr Lustig tell themselves to justify lots of fruit, and yet still be hating on the fructose. Because, of course, fruit is Holy and must NEVER be dissed, so we need to invent stories to retcon their goodness
I don’t disagree that fruit should be used sparingly but there has been a beneficial correlation found between fiber and insulin for women:
https://intensivedietarymanagement.com/fibre-reduces-insulin-how-to-lose-weight-x/