Spanish onions are a large onion, about the size of a softball, in both yellow and red. they have a mild and sweet flavour and can be eaten raw or cooked. Nutrition: Spanish Onions contain Vitamin C making them good for your immune system and general wellbeing. They are also excellent for blood pressure and cholesterol. Storage: Spanish onions can be stored in a cool dark place, such as the cupboard, for up to one month. Do not store them in a plastic bag or in the fridge, as they will deteriorate quicker. Ideal Use: Spanish onions are ideally used in salads - they have a mildly sweet flavour and do not burn when eaten raw like some other onions do.