Home grown onions are an easy item to grow. Even for those gardeners that have a black thumb and kill everything they touch, onions should still be able to live through the touch of death.
The way onions grow, they have several shoots that come off the main root. These shoots develop sugars, which then go into the bulb and help the bulb grow. When the shoots start to die, that is a sign that the sugars are going into the root ball.
When stored properly, most onion root balls can be stored through the winter. In early spring some types of onions will start developing shoots, which is a sign that they should be planted.