Store onions in a cool, dry environment, ideally in a mesh or ventilated paper bag to prevent moisture from collecting around the vegetables. Onions stored properly in a dark pantry or cellar last between two and three months.
To ensure that each onion is kept completely dry for the duration of storage, place the onions one-by-one in a mesh bag or pantyhose material. After each onion is placed in the bag, use a twist-tie to seal off a section of the mesh around that onion. Repeat with each onion, leaving a little space at the top of the bag. If possible, suspend the bag of onions from the wall by hooking the mesh bag onto a screw, nail or other protrusion. This ensures that air flow is maintained over the entire surface of each onion, preventing moisture from causing mold or decay.
For longer-term storage, it is also possible to freeze onions. In this case, peel and slice or chop the onions, then place them in airtight containers or freezer bags. Onions frozen in this manner stay at optimal freshness for up to one year, however they should remain edible indefinitely as long as they are kept at a temperature of zero degrees Fahrenheit or colder.