How to Freeze Onions
To freeze onions, start by peeling and chopping the onions to your preferred size. Be sure to keep the pieces larger than 1/2 an inch to prevent ice from forming on them in the freezer. Then, place the onions in a large, resealable freezer bag in a single, flat layer. Press out all the air before you seal it, and mark the bag with the date you froze them. Place the bag in the freezer and use the onions within 6 months.
Peel and chop the onions. To prepare the onions for freezing, use a sharp knife to cut off the top ½-inch (1.3 cm). Next, slice the onion in half. Peel off the papery skin, and then chop the onion to the size you prefer.. It's not a good idea to chop the onions any smaller than into ½-inch (1.3 cm) pieces because they may wind up becoming encased in ice when you freeze them if they're too small. You can slice the onions instead of chopping them if you're freezing them for use in dishes such as fajitas. Place the onions in a freezer bag. When you've chopped the onions to your desired size, transfer them to a plastic freezer bag. Make sure to place them in the bag in a single, flat layer to prevent them from clumping together when they freeze. Squeeze all the air out of the bag, and seal it. If you're freezing a large number of onions, prevent them from sticking together when frozen by placing them in a single layer on a shallow baking dish. Freeze them like that for two to three hours, and when they're partially frozen, you can place them in the freezer bag without worrying about them clumping together. Be sure to use plastic bags that are thick enough to protect the onions from freezer burn and keep the onion scent contained in the bag. If your bags seem too thin, double them up. Label the bag and freeze. Before throwing the onions in your freezer, use a pen or marker to label the bag with the date, type of onion, and when you should use them by. Place the bag flat in the freezer, making sure that the onions are still in a single layer as you position them. You can keep onions in your freezer for up to 6 months. If you have multiple bags of onion that you're freezing, you can stack them on top of one another to save space in the freezer. Just make sure that the onions in each bag are in a flat, single layer. Peel and chop the onions. Prepare the onions by trimming off the top and the bottom with a sharp knife. Next, remove the papery skin by peeling it off. Use the knife to chop the onions to the size that you prefer. Bring a pot of water to a boil. Fill a large stockpot with water, and place it on the stove. Set the burner to high, and allow the water to come to a full boil, which should take approximately 10 to 20 minutes depending on how much water you're using. The amount of water that you should boil depends on how many onions you're blanching. For every pound (373 g), use 1 (3.8 l) gallon of water. Add the onions to the pot and boil for several minutes. Once the water has come to a boil, place the onions in the pot. Cover the pot, and allow the onions to cook for 3 to 7 minutes, depending on how many you're blanching. The more onions that you're blanching, the longer you should leave them in the boiling water. With chopped onions, it's easier to place them in a wire basket or perforated metal strainer and then set them in the boiling water. That way, you can quickly and easily remove them when they're done. If you don't have a basket or strainer, remove them from the boiling water with a slotted spoon. Transfer the onions to a bowl of cold water. As soon as you remove the onions from the boiling water, place them in a bowl of ice or cold water. Let the onions sit in the cold water for the same amount of time that you boiled them to stop the cooking process. The ice or cold water that you place the onions in after boiling shouldn't be more than 60 degrees Fahrenheit (15 degrees Celsius). While the onions are in the cold water, stir them several times to ensure that they're cooling evenly. Drain the onions and place them in freezer bags. When the onions have cooled long enough, drain them thoroughly in a colander. Shake well to remove the excess water, and blot with a clean towel. When they're dry, transfer them to freezer bags and place them in the freezer. Be sure to label the freezer bag with the date so you can keep track of how long the onions are in your freezer. Peel and cut the onions into pieces. Use a knife to trim off the top and bottom of the onions so you can easily peel off the papery skin. Next, cut the onions into pieces to prepare them for pureeing. You don't need to chop or dice the onion, but cut it into chunks that are small enough to fit in the blender. Use the pitcher from your blender as a guide for how small you should cut your onion. If it's not very large, you should cut the onion into small pieces. In a large pitcher, cutting it into eighths may be sufficient. Place the onions in a blender and puree. When you've cut all the onions into pieces, place them in a blender. Use the puree setting, and hold down on the button until the onions are ground into a thick but smooth puree. If you're freezing a large number of onions, you'll probably need to puree them in batches. Overfilling the blender's pitcher makes it more difficult for the appliance to puree effectively. If your blender's motor isn't very strong, you may need to push down on the onions as you're pureeing to bring them into contact with the blades. To do so, set the handle of a metal ladle through the opening in the blender's lid before placing it on the pitcher. The rounded end of the ladle should be inside the pitcher, so when you start pureeing, you can press down on the onions with the ladle. Because the bottom is rounded it won't get caught in the blades. Transfer the puree to ice cube trays and freeze. Once you've pureed the onions, use a spoon to carefully pour the puree into a clean ice cube tray. Place the tray in the freezer, and allow the puree to fully freeze, which should take approximately four hours. Be sure to cover the ice cube tray with a sheet of plastic wrap to prevent the onion smell from reaching the other food in the freezer. Move the onion ice cubes to a plastic bag and leave them in the freezer. After the pureed onion ice cubes have frozen fully, carefully remove them from the tray. Place them in a plastic freezer bag, and store them in the freezer until you're ready to use them. Remember to label the bag with the date that you're freezing the onions, so you can be sure to use them within 6 months. These pureed onion cubes are ideal for adding to sauces, gravies, and soups. 
Peel and chop the onions. Place the onions in a freezer bag. Label the bag and freeze. Peel and chop the onions. Bring a pot of water to a boil. Add the onions to the pot and boil for several minutes. Transfer the onions to a bowl of cold water. Drain the onions and place them in freezer bags. Peel and cut the onions into pieces. Place the onions in a blender and puree. Transfer the puree to ice cube trays and freeze. Move the onion ice cubes to a plastic bag and leave them in the freezer. 