It is dangerous to cut an onion and try using it the next day because poisonous when kept for a longer time and creates toxic bacteria which may cause adverse effect to your abdomen, and even may cause food poisoning in some cases. The onions kept for more than one day becomes toxic and harmful for health.
You should avoid keeping a portion of a sliced of onion, even you keep it in a zip-lock bag or air tight container its unsafe to use. As per the study, onions are a huge magnet of bacteria.
To get the most benefits of onion, cut it few minutes before consuming, avoid keeping the uncooked onion for a longer time. When consumed fresh, they are a good source of vitamin C, B6, potassium, iron and folate. Onions even provides relief from cold and flu.