Withholding water at this stage helps keep the onions from rotting.
Lay the onions out on the ground or in another open, sunny area for a day or two to dry out the roots.
After a nice day of getting some sun, move the onions into a sheltered, shady spot (under a tree, on a covered porch, or in a well-ventilated garage, for example) and spread them out one by one.
You don’t need to clean off the onions yet. Just set them out to dry, dirt and all, until the stems turn brown and brittle. This rest period allows the onions to go deeper into dormancy so that they’re less susceptible to disease.
The most important part of curing is giving them enough shade and having plenty of air circulation around the bulbs.