Blend chillies (2), garlic (3), ginger (2 1/2 cm), lemongrass (10 cm), onion (1/2), salt (1 tsp) and turmeric (1 tsp) with a little water into a paste.
Heat 2 tbsp oil (2 tbsp) until very hot, fry the paste, stirring so it doesn't burn. Add tamarind (1 tsp) and Thai Chilli Paste (3 tsp); fry, stirring, until darker.
When oil separates, lower heat and stir in coconut milk (300 ml), heating gently. Add palm sugar (2 tsp), sweet soy (1 tsp), fish sauce (2 tsp) and kaffir lime leaves.
When simmering, add fish fillets (4) and poach gently 5 min until cooked, adding water if needed. Top with coriander; serve with steamed rice.