Rinse fish, pat dry, and place in a heatproof, 1-inch-deep nonmetal dish that is at least 1/2-inch smaller in diameter than the pan you will use for steaming.
In a small bowl, mash black beans with garlic; stir in soy sauce and sherry. Drizzle mixture over fish. Cut one of the onions into thirds; place cut onion and ginger on top of fish. Cut remaining 2 onions into 2-inch lengths; then cut lengths into thin shreds and set aside. Arrange clams around fish.
Set dish on a rack in a pan above about 1 inch of boiling water. Cover and steam, keeping water at a steady boil, until fish is just opaque but still moist in thickest part; cut to test (about 5 minutes). If fish is done before clams pop open, remove fish and continue to cook clams for a few more minutes, until shells pop open; then return fish to dish.
Using thick potholders, lift dish from pan. Remove and discard ginger and onion pieces, then sprinkle onion slivers over fish. Heat oil in a small pan until it ripples when pan is tilted; pour over fish (oil will sizzle). Before serving, discard any unopened clams.