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1
Lay the lemongrass and kaffir lime leaves across the base of the steamer rack so that the fish rests directly on the aromatics and the rising steam carries their fragrance up through the flesh from the first second of cooking. Set the rack over vigorously boiling water, and have it at a full rolling boil before the fish goes in — that initial violent burst of steam firms the skin immediately, keeping it intact rather than waterlogged.
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2
Score the fish 3–4 times per side, cutting all the way to the bone so the heat reaches the thickest flesh without overcooking the thinner edges by the time the centre is done. Place it on the aromatic bed, cover tightly, and steam approximately 8 minutes for a 500 g fish or 12 minutes for 800 g; lift the lid and check that the flesh at the deepest point of a score is just translucent — not fully white — because residual heat will carry it the rest of the way off the flame.
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3
While the fish steams, combine the lime juice, fish sauce, and palm sugar in a bowl and stir until the sugar dissolves. Crush the garlic to a fine paste in a mortar, working it until it is completely smooth rather than merely chopped, so the sharp, immediate perfume of fresh-crushed garlic comes through in the sauce rather than the duller, more muted flavour of pre-minced. Add the garlic, sliced chilli, and coriander root to the bowl and leave the sauce entirely raw — this is where the dish lives or dies, because even gentle heat would drive off the volatile lime and garlic fragrance that is the entire point of the dish.
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4
Transfer the fish immediately to a serving plate the moment it comes off the steamer. Pour the cold raw sauce over the hot skin at the table so the contrast in temperature produces a brief, vivid bloom of lime and garlic scent that would be lost if the fish were allowed to cool first. Serve within that transitional moment.