Full sized onions should be removed from the ground, with as much of their wrappers left intact as possible, as soon as they're ready to be harvested. Then either:
Allowed to dry in a dry shady area for a day, then used immediately.
Hung to air dry (cured) for 2 to 3 weeks in a dry shady area with good air circulation until they dry out. To maximise their "shelf-life", they should then be stored in a dry place with good air circulation.
Have a listen to Gardening with Tim and Joe podcast, Dry them off 19 Sep 11 (direct link to MP3), and start listening at 10mins:03secs in.
Full sized onions left in the ground after they're ready for harvesting run the risk of rotting, especially if Autumn (Fall) rain arrives (constantly moist soil) along with the cooling of the soil.
Q. Do you have a garage you could store them in?
Onions can be frozen (for at least 6 months), but then they are only really good for cooking with.
To-do-so:
Clean and peel them.
Cut them into the sizes and shapes you want.
Lay them out on a flat surface (tray) covered with a sheet of wax paper, then place them in the freezer.
Once they have frozen, take them out and transfer them into freezer bags - "portion" sizes, makes using them later on that much easier.
Then put them back into the freezer until you need them...
Another way of freezing onions is to first purée them, then transfer them into "portion" size containers for freezing. Onion purée can be used in things like, soups, curries...
Below are another couple of ways to store (preserve) onions:
If you like Salsa and have the other ingredients necessary to make it...
Pickle them, either by themselves or mixed with other vegetables eg Cucumbers & Onions.