Blend grated coconut (130 g) with turmeric (1/2 tsp) and a little water into a smooth paste.
In a claypot/kadai, mix the paste with enough water plus ginger (1 tsp), chilli powder (1 tbsp), salt, tamarind pulp (3 tbsp) and slit green chilli (1). Taste and adjust.
Bring to a full boil, add fish pieces (300 g) (pre-rubbed with salt, turmeric, lemon juice) and simmer 5–10 min until cooked.
Temper: heat coconut oil (2 tbsp), crackle mustard seeds (1/2 tsp), fry shallots (3) and curry leaves (2) till golden. Pour over curry, mix, cover until serving. Serve with rice.