Peel the onions and cut off any really soft spots.
Slice the onions using the thinnest setting you have on your mandoline to make really thin slices. If you don't have a mandoline, cut the slices as thin as you can. The thinner the slices, the faster they dry.
Next spread the slices out on the dehydrator trays in a single layer.
Place in the dehydrator and set it to 145°F if you have a temperature control. If you do have a model with a thermostat, turn the temp down to 125°F after 2 hours.
Dry time will depend on how thick your slices are, how much moisture is in the onions, and the humidity level. Anywhere from 4 hours to overnight. But they just need to dry until they are leathery.
After they are dry, put them into your blender or food processor. Pulse until powdered.