Onions are a very good source of vitamin C, B6, biotin, chromium, calcium and dietary fibre. In addition, they contain good amounts of folic acid and vitamin B1 and K.
Like garlic, onions also have the enzyme alliinase, which is released when an onion is cut or crushed and it causes your eyes to water.
They also contain flavonoids, which are pigments that give vegetables their colour. These compounds act as antioxidants, have a direct antitumor effect and have immune-enhancing properties.
Onions contain a large amount of sulfur and are especially good for the liver. As a sulfur food, they mix best with proteins, as they stimulate the action of the amino acids to the brain and nervous system.