Peel the onions and chop it as small as you can. If you have a dehydrator, place the chopped onions inside and let them dry for about 3 to 9 hours at 125˚F.
If you don't, spread the chopped onions into a baking sheet and place them in the oven for 1 to 3 hours at 150˚F. You'll know the onions are ready when they crumble easily in your hands.
Remove the onions from the dehydrator/oven and let them cool for about 15 minutes.
Transfer the dried onions into a spice mill, a food processor, or a coffee grinder and grind them up into a powdered form. You can also use a mortar and pestle if you don't have the above options available.
Store the freshly-made onion powder in an air-tight container and place it in a cool, dry place.
Home-made onion powder retains its flavor for up to a year when properly stored.