Opt for firm onions with no mould. The outer skin should be as firm as possible. Onions should be stored in a cool, dry, ventilated area away from light. Storing them in the refrigerator for a long time should be avoided as the odour may be absorbed by other foods. If you do store them in the refrigerator, use the crisper at low humidity. Onions keep up to two months under these conditions.