I regrow green onions on my windowsill. When you initially purchase them, leave about an inch of the bottom white part for the new onion to sprout from. Simply place the onions in a container of water (that you change every few days), and cut green onions as you need them. Then transplant the onions to an outside bed when the weather warms.
Growing onions indoors this way is super-easy and saves us money, plus it’s pretty cool to watch the onions grow so quickly.