Order wrong and had the audacity to blame the customer!

This morning we went to steers in queenswood mall which is a beautiful new mall which we visit frequently. We ordered a bacon cheese chicken burger combo lemon and herb flavour. When we received the burger it had mustard on when the picture on the board shows mayonnaise. I can't eat mustard as I am allergic. When we stated this to the staff they had the audacity to say we are wrong and we should just accept it like that. This was actually said to us by the cook. The manager came out and we pointed out the picture to him which he didn't bother to look at "because he knows his menu". If he knew his menu he would have realized when we asked him which sauce come on this burger and he had the slip in his hand that it's mayo and not mustard. Then he used the excuse the coojs et confused with the beef and the chicken....chicken is white meat beef is red...its not my problem if the staff can't see the difference between beef and chicken or read the slip.After realizing they're mistake he had the nerve to tell me I should tell them to swop the mustard for mayo. I dont understand why I should point something out that is already on the menu? Why should I say no mustard please put mayo on when it has mayo on in the first place? The shop had no one in so they were not busy at all. This is the secone time this happened at the same location and it will be the last. The staff is rude. They claim to know the menu when clearly they don't and if they just apologized and fixed it I would have been happy. This is a disgrace!