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1
Heat the deep-frying oil to 180 °C. Lower the whole fish into the oil and fry for approximately 8 minutes per side, so that the heat penetrates all the way to the bone and the skin has time to blister and set into a rigid shell rather than a soft crust. The fish is ready when the skin is uniformly mahogany in colour and completely rigid — press gently at the thickest point: there should be no give anywhere on the fish.
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2
While the fish fries, heat 3 tablespoons of oil in a wok or saucepan over a moderate flame. Add the sliced garlic and fry until golden, then remove it and set it aside — pulling the garlic out at this point saves it from burning to bitterness during the acid and sugar stage that follows, while the flavoured oil it leaves behind becomes the base of the sauce.
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3
In the same oil, add the sliced bird's eye chillies and fry briefly until fragrant, so the heat draws the volatile oils out of the chilli flesh and distributes their heat evenly through the cooking oil rather than leaving raw chilli bite in the finished sauce.
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4
Add the tamarind water, palm sugar, and fish sauce to the pan and cook over high heat, stirring, so that the water drives off quickly and the sugars concentrate into a lacquer — you are looking for a glossy, thick film that coats the back of a spoon. Reduce too little and the sauce will run straight off the fish and pool on the plate; reduce too far and the sugar scorches and turns acrid.
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5
Return the fried garlic to the sauce and stir to combine, so its toasted savour threads through the syrup at the last moment without softening further. Remove the fish from the oil, drain briefly, transfer to the serving plate, and pour the sauce over immediately before service — the completely crisp skin will hold firm for 3 to 4 minutes after the sauce is applied.