Well, really my first successful experience.

Following instructions I found on the Web, I suspended the onion over a cup of water with only its tip immersed, and when it grew roots I planted it in soil.

Then I waited. And waited. And waited.

The instructions said that the onion would grow roots in less than a week (mine did) and green shoots in less than another week. It took mine about ten weeks. I gave up on it long before that and kept watering it out of sheer stubbornness because it wasn't rotting and it still seemed to have roots.

Now another couple of weeks have gone by; my onion has five shoots, one about four inches long, and it seems to be trying to produce one or two more.

If you try this, be patient. You may have to wait longer than promised, but if you do it right you should get onions eventually. Or you can just go the store and buy some...