-
1
Score the fish deeply on both sides — 4 to 5 diagonal cuts per side at 45° to the spine, cutting down to the bone, so the fish sauce and heat can penetrate the thickest part of the flesh rather than merely seasoning the surface. Season the cavity with fish sauce and tuck in the bruised lemongrass. Pat the exterior completely dry with paper, pressing into every score, because any moisture left on the skin will turn to steam in the oil and both prevent crisping and cause violent splatter — the surface must be entirely dry to the touch before proceeding.
-
2
Bring the oil to 175–180°C in a wok large enough to accommodate the full length of the fish, with oil at a depth of 4–5 cm. Dust the exterior of the fish evenly with rice flour or tapioca flour immediately before sliding it into the oil, so the coating stays dry and bonds to the skin rather than absorbing moisture and turning pasty. Fry for approximately 5 minutes, then turn once using two spatulas and continue frying for a further 5–7 minutes; this is where the dish lives or dies — at the correct endpoint, press the skin surface with a spatula and it must sound hollow and feel crisp rather than yielding, as when tapping a drum, and a skin that still has give needs 2 more minutes before you remove and drain the fish.
-
3
While the fish fries, make the sauce: heat 1 tbsp oil in a pan over medium heat and fry the sliced shallots and garlic until golden, so they lose their raw sharpness and give the sauce a sweet, caramelised backbone. Add the sliced chillies, then pour in the tamarind water, fish sauce, and palm sugar, and simmer for 3 minutes until the sauce thickens sufficiently to cling to the back of a spoon, which tells you it will coat the fish surface when poured rather than running straight off.
-
4
Taste the sauce and adjust the four-flavour balance — sour (tamarind), salty (fish sauce), sweet (palm sugar), and hot (chilli) — until each element is present and no single one dominates. Correct the sourness first, then the salt, then the sweetness, since adding palm sugar last gives you the finest control over where the sauce settles.
-
5
Place the fried fish on a serving plate and pour the hot sauce directly over it immediately before bringing it to the table. Scatter fresh coriander leaves over. Serve within 2 minutes of saucing, because the heat and moisture of the sauce begin to soften the crisp skin at once and the contrast between crackling skin and glossy sauce is the whole point of the dish.