some idea

same thing happened to my friend couple years ago, he found a bug inside of the soup in one restaurant. He tried to phone LC or QC right away in front of the owner, I forgot how it's called, anyway, it's legal department for food quality control. the owner offered several thousand dollars though. I dont know how long have you been keeping that piece of burger, but my friend did call some government function right away.
if you really have a lot of free time, try to sue.
if you don't, talk to the manager and get the refund, maybe there will be some free burger as well.
or just forget it.