Peel the outer layers from the lemongrass stalks and finely chop the lower white bulbous parts, discarding the fibrous tops.
Blend to a chunky paste with the garlic, onion, turmeric and chilli powder in a food processor or grind with a pestle and mortar.
Heat the oil in a large saucepan. Fry the paste for 3 to 4 minutes to release it flavours. Mix in the coconut milk and stock, bring to the boil, stirring. Then reduce the heat, cover and simmer for 15 minutes. Add the salmon, season to taste and then simmer covered for 5 minutes or until the fish is cooked through.
Cook the noodles according to the packet instructions. Drain and divide between four bowls. Meanwhile add the bean sprouts to the fish for 1 minute. Ladle the salmon soup over the noodles and garnish with coriander and lime.
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