Onion

The onion, also known as the bulb onion or common onion, is used as a vegetable and is the most widely cultivated species of the genus Allium

An essential kitchen vegetable, onions are arguably the world’s most widely used ingredient. They vary in shape and colour from flattish brown to round pale bulbs but share a common structure, with a papery outer skin and protecting inner layers of pale, crisp flesh. Yellow onions are hot and pungent enough to make your eyes water; Spanish onions are the largest, with brown skin and a mild, sweet flavour; brown onions are a smaller variety of yellow onion with an even more pungent flavour, making them a good all-rounder. Source: http://www.bbc.co.uk/food/onion