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1
Lay the fish flat on the board and, using a sharp, flexible knife, fillet the flesh away from the backbone on both sides, working from the head toward the tail, so the head, tail, and skin remain in one intact piece that opens flat like a book. Remove the backbone by cutting it free at the head and tail ends and discard it. Both sides of flesh should lie flat, still joined to a single unbroken skin with head and tail connected.
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2
Score the exposed flesh of each fillet in a crosshatch pattern, cuts approximately 1 cm apart, pressing the knife firmly through the flesh down to — but not through — the skin, so the wedges of flesh will fan outward in the oil and lock into the signature squirrel-tail silhouette rather than lying flat and sodden. This is where the dish lives or dies: a cut that breaches the skin will cause the shape to collapse in the oil, while a cut too shallow will prevent the flesh from opening at all. Run a fingertip across the scored surface; the skin beneath should remain taut and unbroken.
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3
Dust the scored flesh, the skin, and the head generously with the 100 g cornstarch, pressing it into every cut so each wedge of flesh is thoroughly coated, because an even, deep coating seals each scored face and gives the individual pieces the hard, separate crust that defines the finished texture. Shake off any excess. The surface should appear uniformly dry and chalky white — no moist patches remaining.
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4
Heat the oil in a deep wok or fryer to 180 °C and lower the fish in flesh-side down, using your hands or a wire basket to hold the body in a gentle upward curve as it enters the oil, so the shape is fixed by the heat before the flesh has a chance to relax flat. Maintain this position for the first minute, then fry for 5–8 minutes in total, basting the fish with hot oil if it is not fully submerged. The fish is ready when the individual wedges of scored flesh have fanned fully outward, each piece crisp and deeply golden, and the fish is cooked through.
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5
While the fish fries, combine the tomato paste, Chinkiang vinegar, sugar, light soy sauce, and chicken stock in a small saucepan over medium-high heat and bring to a boil, then stir in the slaked cornstarch and cook, stirring, until the sauce thickens to a glossy, lightly syrupy consistency that coats a spoon and holds a brief trail. Keep the sauce at a vigorous heat all the way to the moment of service, because a sauce that has sat and cooled will not sizzle when it meets the fish and will soften the crust instead of glazing it.
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6
Transfer the fried fish to a serving platter and carry both to the table. Pour the boiling-hot sauce directly over the fish in front of the diners so the heat of the sauce against the hot crust produces an audible, sustained sizzle — the sound tells you that both the sauce and the fish are at the right temperature and that the crust is still intact.