Peel the onion, trim the top and root end, and chop it to a small dice. You can do this with a knife, but I prefer to use my food processor.
if your hands smell like onions after chopping, wash them and then rub a drop of lemon essential oil on your hands.
Cover the cookie sheet tightly with aluminum foil and set it in the freezer, trying to keep it as level as possible. Make sure the aluminum foil is tightly sealed so the onion smell doesn’t leak out and stink up the freezer.
Leave the onions in the freezer until they’re completely frozen--at least 2 hours. Then take the cookie sheet out of the freezer. See how the onion bits are individually frozen? Freezing them flat on the cookie sheet makes it easy to use and store them.