If you haven't already harvested your onions this year, I want to share with you some information that I found on curing onions properly in a garden magazine called GROW by Fine Gardening. The author of the article, Leslie A. Clapp said that one of the biggest mistakes gardeners make is leaving their onions in the ground too long before harvesting. She said that this invites damage that will result in a shorter shelf life. Harvest the bulbs when they have sized-up and the tops are browning. On a clear, sunny day, pull onions gently from the ground. Do not peel them or rinse them with water because this will also shorten their storage life.
Allow at least a month for post-harvest curing (drying and healing of wounds). She lays her onions out on window screens propped up on sawhorses, out of direct sun, in a well-ventilated spot. The onion bulbs should be spaced fairly wide apart to allow good air circulation; a fan can also be used to help move air around. Turn the onions over periodically, and remove spoiled ones. Once the onion foliage has withered, cut off the tops (leaving an inch of so) and trim the roots. Give the bulbs a gentle brushing and remove some of the outer layers of skin. The onions that look iffy go to the kitchen for immediate use; the others can be hung up in mesh bags for long-term storage.
Hope this information helps. There will be more information about onions in the coming months as you plan next year's vegetable garden. Happy gardening!