To find the best way to store half a leftover onion, we gave the most popular recommendations a try, keeping samples in the refrigerator for two weeks before evaluating the results. Here's what we found:
Onion Stored in Water
A halved onion that was stored submerged in water turned brown (as did the water) and swelled noticeably.
Onion Stored in Oil
Swapping the water for vegetable oil, as some sources suggest, was also a failure; the onion became unmanageably greasy.
The Winner: Onion Stored in Airtight Wrapping
Onions that were stored cut side down—either wrapped tightly in plastic wrap, in a zipper-lock bag, or in an airtight container—showed much more promise.
The cut side dried out a little, so we sliced a thin layer from the cut surface of all three samples before comparing each one, raw and cooked in Simple Rice Pilaf, to samples prepared using a freshly cut whole onion. Tasters found that when eaten raw, the older onion tasted metallic, sour, and harsh compared with the fresh onion. However, in the rice pilaf, no one could distinguish between the fresh and stored onion.
Another Option: Freezing Onion
For longer-term storage, peel and chop the leftover onion and seal it in a zipper-lock bag. When cut, an onion releases an enzyme that reacts with its sulfur compounds to produce its characteristic pungent flavors. Because freezing reduces the activity of this enzyme, it needs to be able to do its job and produce the compounds before the onions go into the freezer, where the flavors will be preserved.
Freezing turns onions mushy, so only use them in cooked applications. (There's no need to thaw them first.)
THE BOTTOM LINE: Store leftover halved onions wrapped tightly in plastic wrap or in a zipper-lock bag or airtight container in the fridge for up to two weeks. Alternatively, chop them and freeze them for up to six months. Be sure to use the stored onions only in cooked applications.
