Wash fully cleaned, scaled fish, rinsing cavity well under running water. Place fish on plate atop steaming rack in a 14-inch wok, or put plate atop rack in covered turkey roaster. Sprinkle soy sauce, sherry, ginger, green onions, and, if used, sesame oil, over fish. Arrange cilantro sprigs on top of fish. Pour boiling water into wok until it comes within 1 inch of fish.
Fish is done when flesh barely begins to flake.
Steam can cause severe burns. Remove plate from steamer and serve fish directly from plate with its juices. Garnish with cilantro and green onion. Makes 4 to 6 servings.