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1
Bring the stock to a gentle simmer in a medium saucepan, then add the galangal, lemongrass, kaffir lime leaves, and bruised bird's eye chillies, maintaining that gentle simmer for 5–8 minutes so the aromatics yield their oils into the broth rather than sitting inert on the surface. The broth is ready when the galangal's piney note and the lemongrass perfume are clearly present in the steam.
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2
Add the mushrooms and simmer a further 2 minutes until just tender. Add the fish sauce and stir to combine, then taste the broth — it should be saline and assertively seasoned at this stage, because the lime juice and prawns added later will temper the salt and you cannot recover a timid broth once they are in.
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3
Reduce the heat so the broth simmers, not boils, before adding the prawns, because a rolling boil will seize them hard and fast, robbing them of the supple texture the dish depends on. The prawns require 2–3 minutes at a gentle simmer and are done when they have curled gently and turned uniformly pink — a tight spiral means they are overcooked and the texture is lost.
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4
Remove the pan from the heat entirely before adding the fresh lime juice, so the citrus stays bright and sharp rather than cooking down to something flat and bitter. Stir once, taste, and adjust fish sauce or lime juice in balance.
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5
Ladle immediately into warmed bowls, distributing the aromatics and prawns evenly. The finished broth should be clear, not opaque, with a sharp citrus fragrance rising from the bowl.