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1
Score the fish with three diagonal cuts on each side, drawing the knife through to the bone, so the aromatics and, later, the sauce have a direct channel into the thickest flesh rather than sitting at the skin. Tuck the lemongrass, coriander roots, and kaffir lime leaf inside the cavity. Set the fish on a steaming rack above vigorously boiling water.
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2
Steam the fish for 8–12 minutes, checking at the minimum time — oversteaming turns the flesh grainy and dull. Open the deepest score mark with a spoon: the flesh should separate from the bone in clean, opaque segments; any translucency means it needs more steam. Confirm with the skewer test: press a thin metal skewer into the thickest point for 3 seconds, and the blade held to the inner wrist should feel hot but not searing.
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3
While the fish steams, warm the oil in a small saucepan over medium heat, then add the garlic and chilli and sauté for 30 seconds, so the raw sharpness of the garlic softens just enough without losing its bite. Add the fish sauce, lime juice, and palm sugar, bring to a brief simmer, then taste: the sauce must be assertively sour and salt-forward, with sweetness only in the background. Adjust as needed and keep the sauce hot over low heat.
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4
Transfer the just-steamed fish immediately to a serving plate, then pour the hot sauce over it all at once — this is where the dish lives or dies: the sauce must be genuinely hot and the fish must not have cooled, because the hot flesh draws the liquid down into the score marks and into its interior rather than letting it pool on the surface. A plume of lime and garlic aromatics rising from the plate confirms the sauce is at the correct temperature.
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5
Scatter the fresh coriander and spring onion over the fish and serve at once.