My first experiment proved to be quite simple and only required a one time purchase of green onions. (make sure the roots are still attached)
After you chop the greens off, put the roots in a glass of water. Change the water every couple of days and you will begin to see new growth.
They will grow just fine in their glass and don't require sunlight, although I have mine sitting on the kitchen windowsill. However, if you feel the need to pot them (as I did with a few below) feel free to do so.
I have three small jars with two or three green onions in each, and 5 plants in one pot. They regenerate so quickly that I don't anticipate ever being in want of green onions again. You may chop and regrow, chop and regrow to your heart's content.
Happy Growing!