Once you've picked your onions a great way to store them is by braiding them in clusters and hanging them in a cool dry dark place making them easily accessible. Just cut one or two down as needed for cooking during the winter. Onions are by far one of my favorite plants we grow, I plant them in the Spring with Gaggi as Jason teaches cooking classes. He always makes sure to mention that if I do it wrong, we'll know come the fall! Well we've got great soil and a green thumb because they always come up huge & sweet - better than anything you can buy! (This may sound like we're crazy but when we travel in Northern Europe in the winter, we always bring our own onions & home jarred tomatoes if we'll have a kitchen - they are just that good, we don't leave home without 'em!)
Once onion tops turn yellow or brown, pull or dig the bulbs on a sunny day, and leave them to dry in the sun. When the outer skins are thoroughly dry, wipe off any soil and remove the tops—unless you intend to braid them. Store in a cool, dry place; hang braided onions or those kept in mesh bags in an airy spot. Such dried bulbs will keep for about 4 months to 1 year.
Step by Step How to Braid Onions & Garlic from OrganicGardening.com