What's the best way to store half an onion?
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. The first sample, which was stored in water, turned brown (as did the water) and swelled noticeably. Swapping the water for oil, as some sources suggest, was also a failure; the onion became unmanageably greasy. However, those 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 cut a thin layer from the cut surface of all three samples before comparing each one, raw and cooked in a 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.
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. Be sure to use the stored onions only in cooked applications.