Onion. The underground bulb of the onion plant, Allium cepa, is the sixth leading vegetable crop in the world. It is thought that onion consumption dates as far back as prehistoric man 1. Onions are a source of vitamin C, vitamin B6, manganese, thiamin, folate, phosphorus and potassium 2. Many of the health benefits of onions are attributed to its sulfur-containing compounds, which are also responsible for the onion’s distinctive odor and its ability to bring tears to the eyes when cut. Onions are also a source of antioxidant flavonoids, such as quercetin 3.