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1
Score the fish with three shallow diagonal cuts on each side, cutting just through the skin to the flesh. This allows steam and aromatics to penetrate without breaking the structure of the fillet. Lay half the ginger matchsticks inside the cavity and half across the scored skin. The fish is ready when every surface carries a thin, even layer of ginger.
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2
Bring a wok or large steamer to a vigorous, rolling boil. Set the fish on a heatproof platter inside the steamer, ensuring steam circulates freely on all sides. Cover tightly. Steam until the flesh at the thickest point — probed with a chopstick — parts cleanly into large, moist flakes with no translucency remaining at the bone. The flesh will be pale white, not glassy.
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3
Remove the platter from the steamer. Drain any accumulated water from the platter — it is dilute and carries no flavour. Scatter the shredded green onion over the fish immediately so that the residual heat begins to soften it. The green onion should smell bright and sharp against the steam.
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4
In a small saucepan, warm the soy sauce and sesame oil together until the surface just begins to shimmer and the sesame fragrance rises. Pour the mixture in a single, even stream over the fish and green onion. The platter will hiss briefly. Serve at once, whole, on the platter: the red skin glistens, the flesh is ivory-white, and the aromatic cloud is clean, sweet, and faintly floral.