1. In a large, deep skillet, bring the water to a simmer with 1 tablespoon of salt. Add the grouper fillets and poach over low heat for 6 minutes, until partially cooked. Using a slotted spoon, transfer the fish to a plate. Pour 4 cups of the poaching liquid into a medium bowl; discard the rest.

2. Pour 2 cups of the reserved poaching liquid into a bowl and add the tamarind; use a potato masher to break up the tamarind as much as you can. Strain the tamarind juice into a small bowl, pressing on the solids; discard the solids.

3. In a small skillet, combine the soaked red chiles, garlic, curry powder, coriander seeds, cumin seeds and belacan and toast over moderate heat, stirring, until fragrant, 1 to 2 minutes. Transfer the seasonings to a spice grinder and let cool. Grind the seasonings to a dry paste.

4. Wipe out the deep skillet and add the vegetable oil. Add the mustard seeds and cook over moderately low heat until they begin to pop, about 30 seconds. Add the spice paste and cook, stirring, until the color deepens slightly, about 3 minutes. Add the onion, curry leaves and lemongrass and cook, stirring occasionally, until the onion is softened, about 8 minutes.

5. Add the tamarind juice to the skillet and bring to a boil. Add the coconut milk, okra, eggplant, tomatoes, Thai chiles, sugar and the remaining 2 cups of poaching liquid and bring to a simmer. Season with salt and cook the curry over low heat until the vegetables are tender and the liquid is thickened slightly, about 40 minutes.

6. Cut the fish into 2-inch pieces and nestle them in the sauce. Simmer over moderately high heat until the fish is just cooked through, about 5 minutes longer.