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1
Rinse the split fish head under cold running water and pat thoroughly dry, so the surface moisture does not dilute the seasoning or puddle on the steaming plate and wash flavour away. Lay it cut-side up on a heatproof steaming plate large enough to hold it flat. The flesh should be pearlescent and smell of clean water — any off-odour disqualifies the fish.
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2
Combine the duo jiao, soy sauce, garlic, and ginger in a bowl and mix briefly, so the soy sauce loosens the fermented chili into a paste that will cling to every fold of the cut surface rather than slide off during steaming. Spread the mixture in an even mound over the entire cut surface of the fish head, covering the cheeks and collar completely. The colour should be a deep, brick-red.
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3
Bring a steamer to a full, rolling boil before the plate goes in, so the flesh seizes immediately and the cheeks and collar set to that silken, gelatinous texture rather than stewing slowly in tepid vapour and turning rubbery. This is where the dish lives or dies: steam over maximum heat without interruption for 8–10 minutes depending on the size of the head. The flesh at the thickest point should be just opaque and yield to gentle pressure without resistance.
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4
Remove the plate from the steamer. Heat the neutral oil in a small pan until it is shimmering and just beginning to smoke, so when it hits the duo jiao the sudden fierce heat opens up the aromatics and drives their fragrance into the topping in a way that gentle oil never would. Pour it in a thin stream directly over the duo jiao topping — a sharp, immediate sizzle signals that the aromatics have bloomed and the final fragrant layer is complete.