Red onions are crunchy, pungent, sweet, and slightly spicy when raw and when cooked their flavor lessens and grows mildly sweeter. The intensity of the pungent flavor varies depending on age, particular variety, and the amount of sulfur in the growing soil or the fertilizer used.

The flavonoids in onion tend to be more concentrated in the outer layers of the flesh. To maximize your health benefits, peel off as little of the fleshy, edible portion as possible when removing the onion's outermost paper layer. Even a small amount of "overpeeling" can result in unwanted loss of flavonoids. For example, a red onion can lose about 20% of its quercetin and almost 75% of its anthocyanins if it is "overpeeled."