If the weather is dry and there’s no danger of frost, the plants can be gently pulled from the soil and laid right in the garden for a day or two. If the weather is wet or frost is possible, move the onions immediately into a protected spot. The floor of the garage or a covered porch works well.
Spread the onions out in a single layer, taking care not to bump or bruise them.
Mild onions should be used up within a few weeks. Pungent onions that will be stored for the winter need to be cured for two to four weeks. Leave them spread out in a single layer. Warm (75-80 degrees F), dry and breezy is ideal. As the onions are curing, their necks will gradually wither and the papery skins will tighten around the bulbs. Once the necks are completely tight and dry, and the stems contain no moisture, you can use scissors to trim the roots off the bottom of each bulb. The leaves can also be trimmed to within 1″ of the bulb. Cull any onions that still have green necks, or have bruised or damaged bulbs. Bring the onions indoors and store them in mesh bags, a bushel basket, or a flat cardboard box with some holes punched in it. Keep the onions as cool as possible (35 to 40 degrees F.) and away from light. A good storage onion kept in a cold, dark place will retain its eating quality for 10 to 12 months.