Add odds and ends and random scraps of any vegetable (that isn’t slimy), even lettuce and the peelings of carrots, but not broccoli or cauliflower since they are too sulfuric. Add parsley or corriander for lots of flavor.
Cover the vegetables with water, bring to a boil, and cook for about an hour with herbs of your choice and a couple of bay leaves.
Strain the broth and freeze in containers. Use broth for the basis of any soup recipe.
If you aren’t ready to cook your broth, just freeze all of these odds and ends (tops of leaks, random tough stems, etc) and then when you have collected enough for a soup, take them out of the freezer and combine with whatever other vegetables you have in the refrigerator.