Unhappy customer!!

I visited burger King Regina Mundi at 20:15 to grab 2 burgers n fries. I ordered a double whopper with cheese plus a big king XXL with jalapeños, no mayo, no onions and 1 packet of medium fries. The cashier that was assisting me advised the person that was making burgers not to put onions n Mayonnaise on my burger, however when I got home there were 3 rings of onions in my burger. I did not want onions simply because I'm allergic to onions... Im definitely not happy with how my burger was made. I took a bite of that onion that was in my burger n I might have a serious reaction because of something that was done on purpose... Im very ****ed because I only eat at burger King n now this!!!!! I'm never ever gonna visit burger King again.