Onions Are Poisonous When Left Over Night. In 1919 when the flu killed over 40 million people there was a doctor that visited farmers to see if he could help them combat the flu. Many farmers and their families had contracted the flu and died. The doctor came upon one farmer, and to his surprise everyone was very healthy. When the doctor asked what the farmer what he was doing that was so different than the others, the wife replied that she had placed an unpeeled onion in a dish in the rooms of the homes. The doctor was amazed and asked if he could have one of those onions. He took the onion and placed it under a microscope and found the flu virus in the onion. It had absorbed the bacteria and kept the family healthy.
Onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. It is not even safe if you put it in a Ziploc baggie and put it in your refrigerator. It’s already contaminated enough just by being cut open and out for a little bit. It can be an extreme danger to you. Especially the cut onions at the hotdog stands!
Also dogs should never eat onions. Their stomachs cannot metabolize the onions and it could make them very ill.
So please remember, it is dangerous to cut an onion and use it or cook it the next day. It becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of the excess bile excretions and even food poisoning.