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.
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.