Why should I put a whole onion in the turkey fryer?
Having done this several times in the past - it is to make the oil flavoured. Like the comments have said that one onion doesn't seem to be enough, and frankly it isn't. I typically use 2, and remove the skins as well as score them allowing the juices to weep out a bit. Additionally, I know some people who toss a de-seeded bell pepper or two in as well, and it turns out well.