Storage is the great thing about onions - there aren't many vegetables that keep as well and taste as fresh as onions do after storage.
Here are a few storage pointers:
The storage basics are:
If you want to braid onions together - an old, effective and attractive way to store them - do it soon after the harvest while the tops are still flexible. You might want to use some twine to reinforce the tops, and be sure to hang the braids in a well-ventilated, warm, shady spot to cure. After the onions have cured, store them in a cool, dark place, and bring out one braid at a time to use. The braids are pretty to look at, and they're a handy way to keep onions. (Garlic can be braided, too.)
Onions are easy to dry, and being lightweight, they reconstitute easily. Peel and slice them in rings about 1/8 inch thick and put them in a dehydrator at about 140°F until they're nearly dry. To keep the pieces from browning, bring the temperature down to 130° F for the last hour or so and keep testing for dryness. If you don't own a dehydrator, try drying onions in your oven. Spread them on a cookie sheet and leave them in a barely warm oven for several hours, checking periodically.
When the onions are dry, remove them from the dehydrator, cool them and store them in sealed containers in a cool, dry place.
If you like snack foods, onion rings that have been French-fried and then dried in a dehydrator are a delicious party treat. They don't store well unless they've been vacuum sealed, but they're so good they won't stay around long enough to need storage!
Onions are so easy to store for fresh use that you probably won't want to bother freezing any. However, if some of your onions aren't keeping well or are starting to sprout, you can salvage them by peeling and pureeing them in a blender. Pour the puree into ice trays, cover them with plastic (so the odor won't affect other foods) and freeze them. After the onion cubes have frozen, transfer them to a plastic bag in your freezer. They're good for gravies and taking the "canned" taste away from canned soup.
If you want to freeze whole onions, however, here's how: Peel and wash the onions and blanch them in scalding water until the centers are heated (three minutes for small onions, seven minutes for medium to large ones). Cool, drain and put the onions on cookie sheets, and place the sheets in the freezer. After they're frozen, put the onions in a plastic bag for convenient storage. Freezing them in this two-step way makes them easier to use; they stay separate, so it's easy to take out only the amount you need.
To keep large, European onions that don't store well, wash, chop and freeze them without blanching. Pack them in small containers, leaving 1/2 inch of headspace.
For the best flavor, use frozen onions within a month or two.
Onions can be canned in a pressure canner, but they discolor and lose their shape. It's easier and more satisfactory to pickle them, freeze them or just store them. Even the "canned" onions you find in the store aren't plain - they're usually pickled with a brine and spices.