Technically no onions, no garlic.
(But then we're not supposed to drink diet coke/pepsi max either, and I've just posted on that thread too.)
The thing is, in SOME people these things will mess up ketosis and make you hungry, which is the only reason why the rules exist.
IF you can get away with having them without dropping out of ketosis and without slowing your weight loss, causing cravings and hunger then it's your choice whether or not you have them. Only you can really decide.
But in the books, well they just have to draw the line somewhere, that's all.
I'm the first one to mess around with the diet and come up with new things for me to try that won't ever be in the books, but I can get away with.
It's personal choice, if however you have onions and garlic and your weight loss slows down or you get hungry - then don't have them.
I'm off for some dinner.
So remember technically no onions and no garlic because of their higher carb content and they may slow down your weight loss - that's all.