Leeks and onions may not seem like the best candidates for freezing -- but in fact, they both do rather well in the freezer. You can keep your frozen onions and leeks for up to six months. Their texture will be different after you thaw them, making them better candidates for cooked dishes -- such as soups, stews or pasta sauces -- rather than raw ones. The procedures for freezing leeks and onions are almost identical.
Peel the papery skin and any thin, dry outside layers off of your onions. Remove the tough, dry outer leaves from your leeks.
Chop the onions into pieces the size and shape you wish to use later. Cut your leeks in half lengthwise, then cut off and discard the top few inches of their leaves. Wash the halved leeks thoroughly, then chop them into smaller pieces if you wish. Discard the root section.
Blanch your leeks if you wish. Place them into a pot of boiling water for two minutes, then transfer them to a bowl of ice water for another two minutes.
Line a baking tray with waxed paper. Arrange your chopped onions and/or leeks on the baking tray in a single layer. Put the baking tray into the freezer and freeze for several hours or until the onions and/or leeks are completely frozen.
Transfer your frozen onions and/or leeks to airtight freezer-safe containers. Use them within three to six months.