Spread the onion slices out on a baking sheet lined with parchment paper.
Place the baking sheet in a preheated oven set to 150°F (65°C) and bake for 6-8 hours, or until the onion slices are completely dehydrated.
Remove the baking sheet from the oven and allow the onion slices to cool completely.
Once cooled, place the onion slices in a food processor and pulse until they break down into small flakes.
Add the salt to the onion flakes and pulse once more to combine.
Transfer the minced onion flakes to an airtight container and store in a cool, dry place for up to six months.