I have always had a problem with green onions. The store sells more than I ever need in a bundle, and then at least some of them end up going to waste because they've gone bad.
And then I discovered a great and simple trick on this blog via Pinterest.
After using the onion, just plunk the end in a glass of water and put it in a sunny window. Couldn't be easier! Even if I haven't used the onion, but it is starting to go bad, I cut off the wilted part and put it right into the glass. I learned that you have to leave some of the green because the white part alone won't successfully grow. The water in the glass needs to be changed everyday or so. I just use scissors to snip some off to when I'm ready to use it. In less than a month they've grown tall enough to use twice and now they are getting big again. They grow so fast, it's awesome.