Can iguana eat onion

Some iguanas may not like eating acidic foods such as tomatoes and it may be likely that the iguana can not properly break down the acid in the food, not a natural food choice.

Occasionally is fine, only if the iguana doesn't have any negative reactions to it. Some people offer these foods for more variety and color.

You can get more information on feeding iguanas at greenigsociety.org (the above is quoted from that site). They have a very detailed food chart there.