Found this on Pinterest and decided to try it out for myself :
Next time you buy green onions from the store, don’t throw the root bulb portions away. Save the bottom bit of the onions and put them in a cup of water on the windowsill.
The onions will grow new plant stems from the root bulb. The post swears you can do this repeatedly, I have a feeling that the bulb will eventually use up its energy stores and your onions will become weak and thready over time… but we’ll see.
This is about a week’s growth from my first cutting. So it really does work. I’ll repeat the process after a second cutting and see if the plants do as well. I just change the water every couple days to keep it fresh and clean.
Try it and see! I might use more green onion if it was readily available like this.
-Belle
Belle of Dirt 
Addendum to this…
I cut the onions off again the other day; as I suspected, they were MUCH less substantial than they were from the store. I think they will continue to get worse as the energy stored in the bulb is further exhausted. I think some dirt and fairly bright light might be necessary to make regrowth from cuttings a true success.
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