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1
Combine the water, bruised lemongrass pieces, sliced galangal, and fish sauce in the base of the steamer and bring to a vigorous boil, so the aromatic steam rising into the chamber begins scenting the fish the moment it is placed inside rather than heating slowly in a neutral atmosphere. The liquid must be producing a strong, continuous head of steam before the fish is introduced.
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2
Assemble the lime-chilli sauce: whisk together the lime juice, fish sauce, and palm sugar until the sugar dissolves, then fold in the minced garlic, sliced bird's eye chillies, and coriander. Taste — the sauce must be fiercely sharp and salt-forward, with the chilli present throughout, because it will be tempered noticeably once it hits the hot fish and any timidity now becomes flatness at the table. Set aside at room temperature and have it fully assembled before the fish goes into the steamer.
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3
Place the scored fish in the steaming dish and lay the reserved bruised lemongrass pieces inside the cavity, so the rising steam carries that fragrance directly through the flesh from within. Set the dish over the vigorously boiling steaming liquid, cover, and steam for 15 minutes for an 800 g fish.
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4
This is where the dish lives or dies. At 15 minutes, insert a thin metal skewer into the thickest point of the fish — directly behind the head — hold it there for 3 seconds, then withdraw it and touch the tip immediately to the back of the hand; the skewer must be noticeably warm but not burning, indicating an internal temperature of 62–65 °C at the spine, which is the narrow window between translucent and overcooked. Confirm at the scored cuts: the flesh should pull away from the bone with no resistance, but the fish must not yet be flaking away of its own accord — if the skewer is barely warm, cover and steam for a further 2 minutes before retesting.
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5
Remove the steaming dish from the steamer. Pour off and discard the accumulated steaming juices from the dish so the sauce you pour over in a moment is not diluted and does not taste waterlogged. Bring the dish immediately to the table.
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6
At the moment of service, pour the assembled lime-chilli sauce in a single motion over the entire surface of the hot fish, so the cold citrus hits the hot flesh and vaporises in an immediate aromatic cloud — you will hear it sizzle and smell the lime sharpen in the air. The sauce runs into the scored cuts and pools in the dish; serve at once.