Place a pan over medium heat and add the chilli oil. Add the shallot, lemongrass, Thai chillies and galangal, then cook for 2–3 minutes until softened and fragrant.
Pour in the chicken stock and bring to the boil. Allow the liquid to simmer until reduced by about half. Stir in the fish sauce, coconut milk and kaffir lime leaves, then remove the saucepan from the heat.
Heat a large frying pan over high heat and add enough oil to lightly coat the base. Season the fish fillets with salt, then place them skin-side down in the hot pan.
Cook for 4–5 minutes, pressing lightly, until the skin is crisp and golden. Flip and cook for a further 2 minutes, or until just cooked through.