Red onions are cultivars of the onion (Allium cepa) with purplish-red skin and white flesh tinged with red. These onions tend to be medium to large in size and have a mild flavor. Red onions are available throughout the year and are high in flavonoids and fibre (compared to white and yellow onions). 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 An onion is a round vegetable with a light brown skin. It has many white layers on its inside which have a strong, sharp smell and taste. Finely chop the onion, and add it to the pan with the garlic.