I read a very interesting story about onion in the internet and would love to share it with you.
In 1919 when the flu killed 40 million people there was a Doctor that visited the many farmers to see if he could help them combat the flu...Many of the farmers and their families had contracted it and many died.
The doctor came upon this one farmer and to his surprise, everyone was very healthy. When the doctor asked what the farmer was doing that was different the wife replied that she had placed an unpeeled onion in a dish in the rooms of the home. It obviously absorbed the bacteria, therefore, keeping the family healthy.
I have read few other stories too where sick people have benefited from keeping onion nearby while they were sick. The onion would attract all the microbes leaving sick person into safety.
Seems like, t is extremely dangerous to consume the leftover half cut onion.