Add up to 4 cups chopped vegetables, including but not limited to leeks, scallions, celery, celery root, tomatoes, fennel, parsnips, corn and carrots. Be sure to add their stems, leaves and roots too. (Avoid brassicas and squashes, which can turn the stock bitter and cloudy.) Additional umami boosters include soy sauce, kombu, dried mushrooms, Parmesan rinds, vegetable bouillon and nutritional yeast. You also can add more seasonings, such as fresh thyme, parsley or sage; dried or fresh bay leaves; coriander or fennel seeds; peppercorns; ginger; and fresh or dried chiles.