1. Peel and slice the onions: Remove the outer skin of the onions and slice them into uniform pieces using a sharp knife or mandolin. You can choose to cut them into rings or use chopped onion, depending on your preference.
2. Spread the thin slices out on the parchment-lined baking sheets in a single layer.
3. Next, place the onion pieces in the oven at its lowest temperature around 130-150ºF/54-66ºC.
4. If your oven doesn’t go as low as that, use the lowest temperature and prop the door open with a wooden spoon handle, so the steam can escape while the onion dehydrates.
5. Check on the onion hourly, flipping it over a couple of times, to dry it faster.
6. Depending on how thin and uniform your onion slices are, you can expect it to take about 6-10 hours to dry onions.

Grinding the Onions into Powder

1. Once cool, the onion slices should snap. If not, put them back into the oven again.
2. Once the dried onions are ready, transfer them into a spice grinder or food processor.
3. For minced onion or onion flakes, you’ll pulse the grinder until it’s at your desired consistencey. For a finer texture and smooth powder, continue grinding for longer periods of time.