See the green onions in the picture above? I grew them!!! And it was super easy! All you need is green onions, a glass, and water! Next time you buy green onions chop off the bottom 2 inches and throw them in a glass of water. After about two weeks they will grow back and look the same as the day you bought them! (You will need to refill the water every few days.)
I have noticed that after the first re-growth they do not grow as fast (I am assuming because of a lack of nutrients since the onions are just in water). If you want to be able to continuously re-grow them, simply plant them in some soil and water regularly. When you plant them make sure that the bulbs (the bottom inch) are covered by soil. Make sure that a little bit of the onion is still poking out of the ground though.
Whenever you want green onions just use a pair of scissors to cut off how much you need! (I harvested mine all at once.)