I was recently asked this question in the garden.
If you know the clues, onions tell you.
When onions are mature, the tops yellow and naturally fall over.
When most of the onion planting has flopsy tops, harvest. To harvest dig around the bulbs, pull them up and cure them by spreading them on a surface where there is good ventilation. I have found using an old window screen as a shelf in the corner of the garage works well.
Let them dry for two to three weeks. You know they are ready when the tops are dry and the outer skin is paper dry, crisp to the touch. Trim off the roots and tops.
If your harvest was abundant and you want to store some for winter, place them in a container with good air flow. Some people use mesh bags but a cardboard box with some holes cut through works too.
If the onions are kept cool, around 40 degrees, and away from sunlight they can last for months.
Gardener’s Tip: While you are harvesting onions, try not to bruise them. They will last longer.