nions are rich in two health-benefiting compounds: flavonoids and sulfur-containing compounds (or alk[en]yl cysteine sulfoxides, if you really want to know the name). Flavonoids are potential antioxidants that could protect us against cancer, heart disease, and aging. They are often found at high concentrations in the skins and outer layers of onions with yellow or brown color (but some are masked by red or purple color). (Read more on the benefits of flavonoids here.) The sulfur-containing compounds are the flavor precursors that give rise to the characteristic odor and flavor of onions, which will be discussed below.