And I'm pleased to find that it actually mostly works.
To grow "free" green onions, you simply cut no lower than a half inch to an inch of the bottom part (the white area with the roots) and place this part in a small container with a little bit of water (I'd say no higher than 1/2 or 3/4 of the plant clipping). Place on a sunny windowsill and be sure to change the water every couple of days. The roots and the greens will begin to grow fairly quickly. By doing this, you can squeeze out a few more uses from these plants without having to by new green onions from the store.
I understand that the whites are delicious. I love them too, but I also love free food and saving money on groceries. I also know that a lot of recipes don't actually call for using the whites (to the point that one friend of mine didn't know that the white parts were edible).
If, however, you must buy green onions, my best suggestion is to go to an asian market to do so. They are usually a much lower price and of a higher quality than the regular grocery stores tend to offer.
What's next? Hopefully celery, carrots, and/or radishes! We shall see. :-)
The above image was found on www.quantum-immortal.net.