1. Set a rack (cake rack, removable rim of a cake pan, or empty 2-in. tall cans with both ends cut out) in a 12-inch steamer, 14-inch wok, 12- by 17-inch roasting pan, or 12-inch frying pan (with sides at least 2 in. tall). Pour water into steamer to about 1/2 inch below top of rack. Cover steamer and bring water to a boil over high heat.
2. Meanwhile, rinse and drain black beans. In a small bowl, with a wooden spoon, mash beans, garlic, and sugar together. Stir in sesame oil, wine, pepper, and 1/8 teaspoon salt.
3. Rinse fish and pat dry. Place in a shallow bowl or on a rimmed plate that will fit inside steamer without touching sides. Sprinkle 1/2 teaspoon salt inside cavity and over outside of fish. Spread black bean mixture inside and all over outside of fish.
4. Set bowl with fish on rack in steamer and cover. (If cover doesn't fit over fish, tent steamer with a large piece of foil, sealing it around pan edges so steam can't escape.) Steam fish for 5 minutes, then remove from heat and let stand until barely opaque but still moist-looking in center of thickest part (cut to test), about 4 minutes longer. If fish isn't done, return to heat and steam 1 to 2 minutes longer. Remove bowl with fish from steamer; if desired, carefully pour off any liquid in bowl.
5. Meanwhile, stack ginger slices and cut into fine slivers. Trim and discard ends from green onion; cut onion, including green tops, into 2-inch lengths, then cut pieces lengthwise into fine shreds.
6. Sprinkle ginger and green onion over fish. Drizzle soy sauce evenly over all. Pour salad oil into a 6- to 8-inch frying pan over high heat; when hot, in about 30 seconds, drizzle evenly over fish. Serve immediately.