Food allergy is real, and some people suffer very serious reactions or even death.
Such as the woman (herself a physician) who recently died at a Disney restaurant.
But in THIS man’s case, the account is vague so far (as it was last time around when he sued a different restaurant).
BTW, onion allergy often will go with garlic allergy.
As well, although restaurants DO have responsibilities, so does the consumer.
If an ingredient cannot be easily seen, that’s one thing—for example hidden milk content or hidden peanut contact.
But onions on a burger might be pretty visible.
I tell my patients with food allergies to be aggressively paranoid.