Didn't need the whole thing, but not sure how save the remaining half. And if it can be saved, for how long?
Put it cut side down on a piece of plastic wrap. Fold wrap up over the sides and twist top to encapsulate the onion. Store in fridge. I have had them last for weeks.
Just as an addition to the tip of davidpx, leave the outer peels on. It helps against possible dehydration. And don't touch the moist part you cut off with your hands. Not of significant importance, but it will increase the life span even more, as it doesn't have any germs on.
When you are ready to use it, take a thin slice of exposed edges that look a little dried out, underneath it will probably be fine.
You could also grate it on a box grater and freeze it.
And if you forget about the half onion and it dries out or you just don't want to use it for cooking, I keep a "stock bag" in my freezer. It contains bones from meat and vegetable odds and ends. When I fill up the bag, I make stock.
Thank you all - what a great bunch of ideas!
I save partial onions all the time, simply wrapped well in plastic wrap in the fridge. As mentioned, if the outer cut edge gets dry, just trim it. Try to use it up within a few days so it has all its flavor - onion is so widely used in endless preparations, figuring out how is easy.
I would just chop it or dice it and save it in an airtight container. I often do that on purpose over the weekend to use during the week. It is a great time saver and avoids oniony hands during the week.
I mean store in the fridge.