Wait for one-half to three-quarters of your crop to flop, then bend over the stems of any remaining upright plants.
When you notice the leaves on the first few plants start to fall over, stop watering and leave the onions in the ground for 7 to 14 days to allow them to finish maturing.
Lay the onions out on the ground or in another open, sunny area for a day or two to dry out the roots.
The most important part of curing is giving them enough shade and having plenty of air circulation around the bulbs.
Once the onion tops and roots have fully dried, you can tidy ’em up by trimming the roots and cutting off the stems with garden scissors.