Heat 1 tbsp vegetable oil in a large saucepan over a medium heat. Fry the ginger and garlic pastes for 2 minutes. Add the red curry paste and allow to sizzle for a few seconds. Then pour in 800ml coconut milk.
Bring the mixture to the boil then reduce to a simmer. Stir a little, and wait for the oil to rise to the surface.
Add 1 tbsp of the fish sauce and pinch of brown sugar. Make sure to taste at this stage. If you like it a little saltier, add more fish sauce; if you like it sweeter, add a little more sugar.
Bring to the boil, then remove from the heat. Add in the Thai basil, reserving a few leaves to garnish later.
Heat 2 tbsp vegetable oil in a frying pan over a medium to high heat. Once the pan is hot, add the cod skin side down for 4-5 minutes before flipping the fish over, cooking on the other side for 3-4 minutes.
Spoon the curry into four bowls and scatter with the chopped red chilli, ginger matchsticks and a few Thai basil leaves.
Notes
The Thai basil leaves can be substituted for coriander if you prefer.