Begin by preparing the ingredients. Peel the prawns, placing the shells and heads in a small saucepan and the prawns into a bowl. Add 200ml water to the saucepan. Bring to the boil, then simmer gently for 15 minutes. Strain, you should have approximately 180ml fish stock.
Prepare the vegetables. Trim and very finely slice the fennel (I used a mandolin). Do the same with the courgette, cutting at an angle. Finely trim the spring onions, leaving the majority of the green part intact. Have the frozen peas ready in a bowl.
If you're unfamiliar with lemongrass, remove the tough outer layer and use the tender inner part. Discard the top and bottom of the stalk. Then finely chop. In a pan, heat a little cooking oil over medium heat. Add the onion (or shallot) minced garlic and lemongrass. Sauté for a couple of minutes until fragrant.
Pour in the coconut milk and fish stock, stir well. Allow the mixture to simmer gently for about 5-10 minutes, letting the flavours meld. With the sauce simmering add the basil and parsley, cook for no longer than 60 seconds, then remove from the heat. Blitz until smooth. Pour back into the saucepan and add the fish sauce. Check the seasoning, add more fish sauce if needed. Gently simmer and add fennel and spring onions, cook for 1-2 minutes or so, then add the courgette and peas. Continue to simmer for 1-2 minutes or until the vegetables are just cooked. Add the prawns to the hot sauce and remove from the heat.
Lightly coat the flesh side of the loin with a little sunflower oil. Season with salt and pepper to taste. In a separate heavy based pan, heat over medium-high heat. Once hot, add the cod and sear for about 3-4 minutes on each side or until they're beautifully golden brown and just cooked through (timings will depend on the thickness). *Option: add a knob of butter halfway through cooking for added flavour and colour.
Portion the glorious green sauce, vegetables and prawns between 2 bowls and top with the seared cod. Serve immediately with a wedge of lime, basil leaves, chilli and an optional side of Jasmine rice.