Heat the oil in a wok or large saucepan on high. Saute the onion, lemon grass and garlic for 2-3 mins until the onion is tender. Stir in ginger, chilli flakes and turmeric. Cook, stirring, for 1 minute.
Blend in coconut milk and fish stock. Bring to boil. Reduce heat and simmer, uncovered, for 20 mins until the sauce thickens slightly.
Add fish. Heat, stirring for 3-4 mins until the fish is cooked through. Remove from the heat, and stir through the coriander, basil and chilli. Cool and serve with the cooked jasmine rice.