Sprinkle the turmeric over both sides of the fish and pat it into the skin. Refrigerate, covered, to allow the turmeric to flavour the fish at least 30 minutes or preferably overnight.

Heat the oil in a large skillet over medium-high heat. Add the fish and cook until they are seared on both sides, 3 to 5 minutes per side. Transfer to a plate.

Add the onion, chiles, and tomato to the skillet. Scrape the bottom to deglaze it, releasing any browned bits of fish. Lower the heat to medium and cook, stirring occasionally, until most of the liquid from the tomato has been absorbed and the sauce appears dry, 5 to 8 minutes.

Pour in 1 cup water and bring the curry to a boil. Cover the skillet, reduce the heat to medium, and cook until the onion is tender when cut with a fork but still looks firm, 10 to 12 minutes. Stir in the cilantro.

Dissolve the tamarind in the ½ cup water. Stir the coriander, salt, and garam masala into the onion mixture. Pour the tamarind liquid into the skillet and raise the heat to medium-high. Cook, uncovered, stirring occasionally, until the sauce thickens, 3 to 5 minutes.

Return the fish to the skillet, and spoon the onion and sauce over them. Cover the skillet and poach the fish over medium heat, turning them over once and covering them again with sauce, this should take 5-10 minutes in total. Serve immediately with rice