To freeze onions, peel dry layers off the onions, slice them or chop them into medium-sized pieces, and place them into a resealable bag. After that, mark the date on the bags, and write that they contain onions, if needed. Place all the bags onto a metal tray, and freeze them. Leave frozen onions in the freezer until you need to use them.
When placing onions into plastic bags, leave about 1/2 inch unfilled at the top of each bag. Press the bags down before sealing them completely to get rid of the air inside. Placing small amounts of onions into multiple bags, so that the bags are flat, makes it easier to break off onion pieces. The onions also freeze faster when this technique is used.
To use frozen onions, break off several onion pieces, and add them directly to the dish as you would add fresh onions.
Frozen onions are safe to use for about three to six months after they have been frozen, but they taste the best if you use them within six weeks because they lose their flavor over time. Avoid placing onions directly next to delicate foods in the freezer, as the flavor of onions transfers to other foods easily.