First make the soup, which can be stored in a covered container in the fridge for up to one day.
Heat a large heavy-based saucepan over a medium-high heat, then add the oil. When it is hot, add the spring onions and garlic and sauté for 1 minute.
Now add the bananas, chilli, fish sauce and sugar, and season with salt. Continue sautéing for a further minute, then stir in the coconut milk and vegetable stock and bring to the boil. Reduce the heat and leave to simmer for 5 minutes.
Transfer the soup to a blender or food processor and blitz until smooth. Pass the soup through a fine sieve, then leave to cool completely and chill if not using immediately.
When you’re ready to cook the fish, preheat the oven to 180°C Fan/200°C/Gas Mark 6. Pat the cod fillets dry and season with salt.
Heat a well-seasoned ovenproof sauté or frying pan over a medium-high heat, then add a splash of oil. When it is hot, add the cod, skin side down, and fry for 2 minutes.
Put the pan in the oven and roast the cod for 5 minutes, then gently turn the portions over and roast for a final minute, or until the flesh flakes easily.
To finish, reheat the soup and adjust the seasoning, if necessary. Thinly slice the uncooked banana just before serving.
Divide the soup among four bowls and add a cod portion to each, then garnish with the banana slices, chilli, spring onions and coriander. Add a squeeze of lime to each bowl and serve.