Asked for no Garnish

I bought 4 wacky Wednesdays this week and i asked the lady at the till so nicely that I don't want onions on the one set of burgers she said okay, when i received my order everything had onions on there wasnt one burger without onions im highly allergic. I only realised this when i came home. I couldn't go back as i didn't have transport. I really love steers but it's service like this where you ask the staff so nicely and they still just don't do what i pay for. Deeply disappointed