Choose storage onions (yellowskins, Spanish and reds) that are dry on the outside and firm to the touch. The skin should be papery, almost brittle, and the shape smooth, round or oval. Store in the mesh bags they're sold in for maximum air circulation. If kept cool, dry, and in the dark, they should last well for up to one month.

Bunching or green onions are meant to be used fresh. Look for smooth, firm, white bulbs and bright green tops. Trim immediately and refrigerate in a plastic bag for up to one week.