Slice onions into ⅛- to ¼-inch pieces and place on dehydrator racks without overlapping.
Dry onions in a food dehydrator on 37 to 54°C until completely dry. The slices should snap and crack crisply in half, and not bend.
Alternatively, to dry onions in the oven, spread them in a thin layer on a baking pan covered with parchment paper. Bake at 60°C until they are completely crunchy dry.
Once fully dried, place the onions in a blender or food processor and pulse until the desired powder coarseness is achieved.
Transfer the ground onion powder into an airtight container and use within one year.