Yes! We love keeping parsley stems, the soft outer peel of the onion, browned ends of celery stalks and green beans in a plastic bag in the freezer. We add them to the stock in step 2 with the other veggies. It is important to make sure they are clean and it's best to avoid bitter veggie scraps such as carrot peels and celery leaves. Limit the strongly flavored vegetables (such as fennel) and brassicas (like broccoli and brussels sprouts), as they will add off-flavors to the stock.