When selecting onions they should be firm for their size and have little to no scent. Avoid bulbs with any cuts, bruises, or blemishes. When purchasing whole peeled onions, select ones with an outside layer that does not show signs of being dehydrated.
Do not wash onions that will be stored for a long period of time. Washing the onions can actually cause spoilage. Trapped moisture will result in mold and premature deterioration. If you will be storing onions for a few days or using right away you can peel off the first layer or two, cut off each end, and rinse under cold water.
To properly store them for a long period of time, gently brush away the dirt with a soft dry cloth. When storing onions, select a cool, dry, dark location such as the bottom of a pantry. If you are storing them for a few days after cleaning, gently dry the onion, place it in an air tight bag or container, and place in the refrigerator until you are ready to use it.