Do you have an actual Green Iguana, scientifically named "Iguana iguana"?
I forget the original reasons, but for decades we have discouraged feeding any parts of onion of any kind.
Somewhere on the internet, there is a list of greens for iguanas in the Far East, such as the Phillipenes. I remember one that was called Water Spinach. In those days there was more traffic from the Far East, and the information you need circulated more.
Hopefully others will add to my meager effort.