The best way to extend the shelf life of your onions while maintaining texture is to store them in a cool, dry place. I typically store my onions in my pantry in a covered basket. You can also store onions in the legs of clean, sheer pantyhose by tying a knot between each onion and hanging in a cool, dry place (like your pantry). When you are ready to use an onion, all you have to do is cut above the knot to grab an onion.  Freezing sweet onions is a possibility if you chop and place on a cookie sheet to freeze. Once frozen, you can throw the onion pieces into a freezer bag until you are ready to use.