What are the nutrients contained in onions?

Several: Onions, the big bulbs, are mostly water, with some carbs, fiber, a few vitamins and minerals, a dab of protein and very little fat (oil). They are very low in calories and add flavor to foods. See https://en.wikipedia.org/wiki/Onion There have been a few small clinical studies that suggest more specific benefits of onions. See http://healthtools.aarp.org/natstandardcontent/onion/2