Red onions (also known as purple or blue onions in some mainland European countries) are cultivars of the onion (Allium cepa), and have purplish-red skin and white flesh tinged with red. They are most commonly used in cooking, but the skin has also been used as a dye.Red onions are full of sulfur compounds that protect the body from ulcers and various cancers. They can also fight bacteria in the urinary tract. The most important of these compounds is called quercetin - an antioxidant compound that could provide protection against cancer, heart disease and allergies.
