If you like to cook, you already know that onions are a vital part of any recipe. Despite their eye watering reputation, onions are a nutritional super food. Onions are high in vitamin C and are also a good source of dietary fiber and folic acid. They are also a major source of a flavonoid called quercetin. Flavonoids are very similar to antioxidants and are often classified as such. Cooking onions will diminish some of the beneficial properties, unless they are being simmered in a liquid, such as soup.