To make green onion flakes, pulse the dried pieces in a blender/food processor (or crush them with a mortar and pestle) to your desired consistency. For a smooth green onion powder, it's best to use a coffee or spice grinder to do the job. Note for every ¼ cup of raw green onions, you'll need a tablespoon of dried or about 1 ½ teaspoons of green onion powder.