Man you are making my toddler seem not fussy with food! 
But I am not really against you asking for specific extras not to be on the food. I note more places these days tend to do things on the side I guess for that specific reason. Personally I don’t like raw onion in things. But I would never bother to say that, I just pick it out of the salads or pull it off the burger before I eat.
However, this obsession with giving everyone onion breath as oppose to just using a relish is why I’ve not been able to eat a cheddar cheese based sandwich from Pret for probably a decade. They seem to think cheddar cheese without a raw red onion on it is a mistake. FUCK THEM. (They also put them in their falafel wrap. Fuck them.)