Red onions also known as Spanish onions, contain plenty of healthy essential nutrients as well as phytochemicals that provide long-term health benefits. Including red onions in your daily diet may help reduce your risk of chronic disease. How you prepare red onions can affect the overall health benefits you derive from them. Cooking red onions can break down some of the nutrients and phytochemicals, but cooking red onions over low heat instead of high heat can reduce nutrient loss. Antioxidant compound that may offer protection against cancer, heart disease and allergies in an onion are concentrated near the outer layers, so peel as little of the outer skin as possible to reap the most benefits.