Steaming sea bass keeps its richlayers of flavor intact. An addedbonus is that it's very low in fat.
Rinse fish thoroughly under cold running water; remove any debris from the cavity with a spoon; pat dry. Season cavity with salt and pepper; stuff with ginger slices, 4 slices garlic, and the cilantro sprigs. Rub fish with 1 1/2 tablespoons sesame oil. Season with salt and pepper.
Put bok choy, mushrooms, two-thirds of scallions, remaining garlic, the grated ginger, and 1/4 teaspoon salt in a medium bowl. Season with pepper, and toss.
Transfer half of bok choy mixture to a 9-by-13-inch baking pan; place fish on top. Top with remaining bok choy mixture. Whisk together soy sauce, vinegar, mirin, and 1/4 teaspoon salt; drizzle over fish. Tightly cover pan with foil.
Pour water to a depth of 1/4 inch in another 9-by-13-inch baking pan; bring to a boil on top of stove. Reduce heat; let simmer. Set pan with fish on top; steam until cooked through, 16 to 20 minutes. Transfer to a platter; garnish with remaining scallions and chopped cilantro. Drizzle with remaining 2 tablespoons sesame oil.