Disambiguation
If a recipe calls simply for an 'onion', you can likely assume it means a yellow onion. The ingredients listed under 'onion' here are not strictly onions by the botanical sense, but in the culinary sense. Onions aren't botanically the root either, but are often considered to be root vegetables.
Description
Onion plants have a fan of hollow, blue-green leaves, and develop a bulb when maturing. This bulb is the 'onion' in the culinary sense, and is composed of layers of compressed, modified leaves that surround a central bud. Mature onions are harvested when the foliage has died and the outer layers of the bulb have become more dry and brittle. They are then allowed to dry further to improve their storage capabilities. Onions are also harvested before being fully mature (i.e. spring onions). In Buddhist temple cuisine, onions are one of the five forbidden vegetables, considered a stimulant that hinders spiritual meditation.