Preheat oven to 150 degrees. (or use a food dehydrator).
Spread the very finely minced onions out on baking sheets. Place in the oven and Scrape and loosen the onion bits several times as they are drying. (Humidity and the amount of moisture in the onions is what will determine how long it takes them to dry, so I am not giving a time... other than hours).
The onions are dry when you can easily crumble the minced pieces with your fingers. Remove from oven. Let cool.
When completely cooled, place half of the onions into an airtight container until you have a recipe that calls for dehydrated onions.
Grind the remaining onion bits by hand or using a spice grinder, coffee grinder or food processor. Take the ground onion powder and store it in an airtight container until needed.
****The amounts that you get depends on the size of the onions and moisture content of them.
*** This is a great recipe to start in the evening or very early in the morning when you don't need to use your oven.
*** I also do this with green onions to make chives.