Onions are known to be rich in biotin. Most of the flavonoids which are known as anti-oxidants are concentrated more in the outer layers, so when you peel off the layers, you should remove as little as possible. Onions, like garlic, are also rich in sulphur compounds. Onions are known to contain manganese, copper, Vitamin B6, Vitamin C and dietary fibers along with phosphorus, folate and copper.