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1
Score the fish on both sides in a criss-cross pattern, cutting all the way to the bone at intervals of approximately 2 cm. Insufficient depth here prevents the flesh from fanning open and the tail from arching during frying.
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2
Dust the entire scored surface of the fish generously with cornstarch, pressing it into every cut so that no moist flesh is exposed. The coating should appear uniformly dry and white.
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3
Combine the vinegar, sugar, soy sauce, and dissolved cornstarch in a bowl and set beside the stove. The sauce must be fully prepared before the fish enters the oil — timing between frying and saucing is critical.
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4
Heat the oil in a deep wok or vessel to 180 °C. Hold the fish by the head and tail, lower it into the oil at an angle to set the scored shape in the first moments of contact, then submerge fully. This is where the dish lives or dies: at correct temperature the cuts fan open immediately, the body curves, and the tail arches clear of the oil in the 'fish jumping in the river' presentation. At lower temperatures the shape will not set and no arch will form.
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5
Continue frying, basting exposed surfaces with hot oil, until the entire fish is deeply golden and the flesh at the thickest part near the backbone is just cooked through — a skewer inserted should meet no resistance and withdraw clean.
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6
Lift the fish, drain briefly, and transfer immediately to the serving dish. Pour the prepared sauce into the now-empty wok and bring to a boil over high heat, stirring constantly until the starch thickens it to a glossy, lightly viscous consistency that coats the back of a spoon.
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7
Pour the sauce over the fish at once and carry the dish directly to the table. The sauce must contact the fish while both are hot — any delay allows the crust to soften and the theatrical crispness is lost.