Onions are an excellent source of fiber, vitamins, and minerals. Many onions are high in quercetin, which has been associated with a lower risk of developing lung cancer. Onions also contain organic sulfur compounds. Both organic sulfur compounds and quercetin are known to promote insulin production, which may be helpful for those with diabetes. Onions are also a great source of vitamin C, vitamin B6, potassium, manganese, and copper. Although eating onions won’t magically cure illnesses, there are many nutritional benefits that make them an excellent addition to your diet.