But, do not soak onion bulbs in water. How to Cure Onions: Curing onions is easy. It is important to let them dry well. Onions that have too much moisture, can mold and rot in long term storage. Allow clean bulbs to dry in the sun. Spread onions out on a flat surface, in full sun. Allow them to dry in the sun for a few days, until the onion roots and the plant itself is a dry, brown. If rain is in the forecast, cover them or bring them indoors, until the weather clears. Cut the tops off the onion, to about an inch from the bulb. Trim off dried roots.