Preheat the oven to 170°F and line two baking sheets with parchment paper.

Peel each onion, and, using a cheese grater, grate each onion. This should yield about 4 cups.

Transfer the grated onions to a fine wire mesh strainer that is over a bowl and squeeze out as much liquid as you can by gently pressing the onions with a spoon or your hands. (I was able to get about a cup of liquid.) Store the liquid in a sealed container, in the refrigerator, for up to two weeks.

Divide the onions in half and lay them out in a thin layer on each of the parchment-lined baking sheets.

Bake for 5 hours, or until completely dry. The onions will turn pink; this is normal. Please note that baking times may vary. You know the onions are dry when they crumble in your hand.

Let the dried onions cool completely before grinding them.

To grind the onions, place them in a food processor, and grind them down to your desired consistency. You can also use a mortar and pestle for an even finer grain.

Store the powder in an airtight container with a few grains of uncooked rice added to the powder to absorb any excess moisture.