Peel and slice onions thinly
Spread the onion slices in a single layer on a dehydrator tray or baking sheet
Dry the onions until completely brittle and crisp
Let the dried onions cool fully
Grind the dried onions into a fine powder using a blender, spice grinder, or mortar and pestle
Sift the powder to remove any larger pieces
Grind the larger pieces again if needed
Store the onion powder in an airtight container
Keep the container in a cool, dry place