If you dice onions, celery, peppers, etc. and spread them on cookie sheets to place in the freezer, they'll freeze quickly and then can be poured into bags for storage. With this method, they won't all stick together, so just the needed amount can be poured out when used.
Yes, the frozen forms of these veggies are suitable only for cooking, but they'll hold together enough as to be recognizable in the finished dish.
I also throw the top leaves of celery stalks in a container in the freezer and use them to flavor soups, stews, etc.