We harvested the greens from the green onions in a baggie in the window, and it worked so well, we are trying a new setup with onions in test tubes.

I ordered a set of plastic test tubes and a test tube rack through Amazon. I chose the plastic because there is a good possibility of the cat walking on the window sill and knocking it over. We cut off (and used) the green parts of the onion (which my eldest likes best anyway) and put the white part and roots in the test tube, then filled the water just to cover the white part. The idea is that we harvest the greens when they are long enough. After three days, we are already seeing growth!