Add onions to a jar and pour vinegar over. Put the lid on tightly and shake the jar to mix the onions with the vinegar. Place in the fridge and let the onions marinate for at least one hour.
The onions will soften and lose their sharp pungency as they marinate. If some of your onions still look raw after a few hours, add more vinegar and mix well.
Notes
The onions will release liquid as they marinate, so the vinegar shouldn't cover all the onions. They should be about half covered.