Onions have a very dramatic way to show that their bulb is mature, one that may be alarming if you’re not forewarned. Their leaves suddenly flop over, lying on the ground, as if the plant had died. But the onion is not dead: this is only one step in the bulb’s maturation process.
Traditionally, when the leaves of about half the onions in a plot had collapsed, gardeners used a garden rake to knock the others over. This was believed to accelerate their maturation, although it’s far from sure this actually speeds things up.
At any rate, after a week or two after most of the onions have flopped and their leaves are really starting to do downhill, dig up the bulbs up and leave them lying exposed, roots and all, to the sun for a few days. (In case of rainy weather, lay them on a sheet of plastic in a garage or shed.) This cures them (hardens then off), causing the outer layer of dried leaves to thicken. That way, they’ll store better later. Since bruised or damaged bulbs won’t store well and will soon start to rot, don’t bother trying to cure them. Just use them fresh over the following few weeks.
The final step is to clean the bulbs lightly with a brush, bring them indoors and store them in a cool, dry place, like a root cellar. You’ll have homegrown onions all winter!