This is a foolproof way to steam fish. You can use striped bass, black bass, Mediterranean sea bass or even branzino; ask your fishmonger to remove the bones but leave the head on. You can buy one large fish or several smaller ones. The cooking times will be shorter for smaller fish, but use the method in the recipe to check for doneness. Fermented black beans are available in Chinatown, but in a pinch you can use black bean paste, which is available at most supermarkets.