Heat the oil in a wide, lidded frying pan, then soften the onion for 5 mins. Increase the heat a little, stir in the ginger, garlic, chili and lemongrass, and cook for 2 mins. Add the spices and sugar, and continue stirring. When the sugar starts to melt and everything starts to clump together, add the coconut milk, bring to a simmer.
Add the fish to the sauce, tuck the shrimp in here and there, then squeeze over half the lime. Pop on the lid and simmer for 5 mins more or until the hake is just cooked and flaking, and the prawns are pink through. Taste for seasoning, adding a squeeze more lime to the sauce if you like. Scatter over the cilantro leaves and serve with rice or naan.