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1
Bring the fish to room temperature before proceeding, so that the oil temperature does not plunge on contact and leave you with a pale, greasy skin rather than a cracking one. Score the fish deeply on both sides — cuts should reach the bone — at intervals of approximately 2 cm, so the heat drives straight to the centre and the skin lies flat in the oil rather than blistering and lifting unevenly.
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2
Heat the oil in a wok or deep vessel to 180 °C, then lower the fish in carefully. Fry, turning once, until the interior reaches 65 °C and the skin is uniformly deep golden and rigid to the touch — that rigidity tells you the crust has set and will hold its ground when the sauce hits it. Remove and rest on a rack for 3 minutes so the retained heat finishes the flesh gently while the skin tightens further, becoming harder and more resistant to the sauce than it was the moment it left the oil.
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3
While the fish rests, build the sauce: combine tamarind water, palm sugar, and fish sauce in a clean wok or saucepan over medium-high heat, then add the garlic, shallots, and chilli. Reduce, stirring, until the sauce coats the back of a spoon and falls in slow, heavy drops rather than running freely — this is where the dish lives or dies, because a sauce left too thin will run straight off the skin, dilute the crust, and leave the sour, sweet, and salty notes tasting separate rather than reading as one. Once it reaches that slow, reluctant drip, pull it immediately from the heat so it does not tighten further into a sticky glaze that clumps rather than coats.
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4
Transfer the rested fish directly to the serving plate and pour the hot sauce over it immediately, so the heat of the sauce melds with the crust rather than sitting on top of it as a cold, inert layer. Serve within 3–4 minutes of sauce contact, before the zone where sauce meets skin softens all the way through and the contrast between the crisp exterior and the yielding flesh is lost.