Peel and chop, dice, or slice onions to whatever size you most commonly use in cooking. Work in a well-ventilated area or near a fan to minimize tears.
Flash freeze: spread chopped or sliced onions in a single layer on parchment-lined sheet pans. Freeze until solid (1 to 2 hours), then transfer to freezer bags and press out air. The pieces stay individually frozen and pour freely from the bag.
Alternatively, pack measured portions (½ cup or 1 cup amounts) directly into small freezer bags without flash freezing. They will freeze into a clump, but you can break pieces off as needed or use the whole bag at once.
Caramelized onions also freeze beautifully. Cook a large batch of caramelized onions, cool, and freeze in ice cube trays or small portions. They add instant depth to any dish.
3 to 6 months. Onions can become stronger in flavor over extended freezing.
Cook directly from frozen. Drop into hot pans, pots, or slow cookers without thawing. They soften and cook just like fresh onions, though they will not have the same crunch if used raw.