Next time you have green onions, don't throw away the white ends. After cutting off the dark green tops, submerge the white part with roots in a glass of water and place them in a sunny window. Your onions will begin to grow almost immediately and keep you in a nice supply.
Use kitchen scissors to cut what you need; wash and pat dry with a dish towel.
I periodically empty out the water, rinse roots off and give them fresh water. It's like magic!