Lay chopped onions evenly on a baking sheet. Do not let onions overlap. Ideally do not let the onions touch. When frozen individually, they will freeze best, and they will be best to use.
You can place plastic wrap over the onions, and add another layer if desired.
Wrap the baking sheet with onions with plastic wrap and place it into the freezer.
When the onions have frozen completely, divide onions into small portions. I like to freeze mine in 1 and 2 cup portions.
Place chopped onions into small plastic bags. Date the plastic bags and put the bags into the freezer.