Boil plenty of water in a steamer.
Slash (score) the fish (see tips
Stuff the cavity with half of the garlic and ginger, and some pepper.
Rub half of the soy sauce and citus juice all over and inside (the cavity from where the intestines have been removed) the fish.
Lay the fish on a heatproof dish. Pour the remaining soy sauce and citrus juice over and around it. Sprinkle the remaining garlic and ginger. Crack more black pepper over it.
Steam over briskly boiling water for about 20 minutes.
Garnish with onion leaves. Heat the sesame see oil until smoking. Pour over the fish. Serve at once.