Cut the onion. Onions freeze best when cut. It’s best to cut into a medium or small dice, which is versatile for many types of recipes.
Place the onion in an airtight container or freezer-safe plastic bag. If using a bag, use a straw to suck any remaining air out of the bag and seal it. If using an airtight container, you can place a small piece of aluminum foil on top so the onion smell doesn’t permeate the plastic lid.
Mark the “Use by” date. Use a permanent marker to label the onions with a “use by” date that is 3 months in the future.
Freeze. Place the onions in the freezer and freeze for up to 3 to 6 months. For best results, use in 3 months. You can use frozen onions directly in cooked dishes; no need to thaw (do not use raw).