The best way to increase the shelf life and keep onions fresh is to store them whole. Place them in a well ventilated cool dark place. Onions will start to sprout and rot if exposed to too much moisture and light.

You can store them by tying their stalks to rope or string and hanging them. You can even put onions in plastic mesh tubing or old tights/pantyhose. Once, placed in these, tie a knot around each individual onion to allow the maximum amount of cool air to flow around each of them.

Do not keep them in a plastic bag as this is not breathable. Doing so will cause moisture to gather in the bag and speed up the rotting process. Alternatively, you can keep them in a woven basket.