Over at hello its me there is a post today similar to the one I had planned. Remember my post, Celery--free plants to grow ? Well, onion technique is similar to the celery technique. Only, you don't need celery roots. I would suggest maybe just cutting the tops of the onions as they grow, sort of like cutting basil and leaving the onions in the soil. Maybe not!
Either way, you will buy fewer onions by replanting the onion with roots. And, you can raise it for yourself and make yourself more self-sufficient.