Okay, seriously. If you have a serious onion allergy, wouldn't you check the sandwich before eating it? It's not like they're cooked into the meat or something, they're just sitting there, in the sandwich, easily visible if you remove the bun. While I get being mad that you were served something you're allergic to that you asked not to be, this seems like it was an easily solveable problem especially since it happened before.
Unless, you know, he's just going from fast food restaurant to fast food restaurant waiting for one to serve him onions so he can get a payday.