Cut pale part of lemongrass into 5 mm pieces and discard the rest.
Pulse lemongrass, ginger and garlic to a coarse paste in a food processor.
Cut spring onions into 1 cm slices.
Put fish, spring onion, lemongrass paste and chilli into a saucepan with coconut milk and water.
Stir in turmeric, then simmer, uncovered, for 10–15 minutes until cooked, adding lime leaves for last few minutes.
Add fish sauce to taste and serve with jasmine rice and steamed greens.
