Put the oil in a large pan over a high heat. Add the garlic, ginger and shallot and fry for 30secs-1 min until just starting to turn golden brown. Add the curry powder, turmeric and chilli, and stir-fry for 30 secs.
Add the tomato puree, coconut milk, stock, sugar, lemongrass, bay leaves, and fish sauce. Bring to the boil, then simmer until thickened.
Add the cod and prawns, and bring back to the boil. Cover and simmer for 5 mins, until the cod is cooked through and the prawns are pink. Stir in the lime juice, garnish with coriander and chillies, if using, and serve with rice.