Take 60g tamarind pulp and put it in a bowl with 120m1 water. Leave to soak for 15 minutes, then work the paste and strain the slightly syrupy mixture through a fine sieve.
Heat 60m1 oil in a heavy-based saucepan over a medium heat. When hot, add 1 tbsp mustard seeds and fry for 30 seconds, then stir in 1 large chopped onion and 3 cloves of garlic and fry gently for about 10 minutes until softened and lightly golden.
Add few curry leaves, 2 tsp chili powder, 2 tsp coriander and 2 tsp turmeric and fry for 2 minutes, then stir in 2 chopped tomatoes, the tamarind liquid, 2 sliced green chilies and salt to taste.
Simmer for 10 minutes until rich and reduced. Add 700g cut snapper fillets fish, cook for 5 minutes, and serve with plain rice.