In a small bowl, place the black beans and cover with warm water. Stir the beans for about 2 minutes to remove the excess salt. Remove the beans from the water, rinse, and drain well.

Slash the fish crosswise 3 times on each side. In a small bowl, mix the ginger, vegetable oil, black beans, garlic, salt, sugar, soy sauce, and sesame oil; rub the cavity and outside of the fish with the mixture. Cover and refrigerate 30 minutes or longer.

Cut the green onions into 2-inch pieces; shred lengthwise into fine strips. Place in a bowl with ice water to cover; let stand 10 minutes, or until the strips curl. Place the fish on a heatproof plate (see Notes). Place the plate on a rack in a steamer, cover, and steam over boiling water until the fish flakes easily with a fork, about 10 minutes. (Add boiling water if necessary.) Garnish with the green onions.