Goan-Style Fish Curry

Pat the fish chunks dry and season them with 1/4 tsp turmeric and 1/2 tsp salt. Set aside to marinate briefly while you prepare the curry base.
Heat the oil in a wide, shallow pan or skillet over medium heat. Add the mustard seeds and let them pop for 20-30 seconds; cover with a splatter screen or lid briefly if they jump.
Add the curry leaves and let them sizzle for 10 seconds until fragrant and slightly crisp.
Add the chopped onion and cook, stirring often, for 6-7 minutes until soft and light golden.
Stir in the ginger-garlic paste and cook for 30 seconds, stirring constantly, until fragrant and no longer raw-smelling.
Add the chopped tomatoes and cook for 5-6 minutes, mashing occasionally, until they soften and the oil begins to separate at the edges.
Add the remaining 1/4 tsp turmeric, chili powder, and ground coriander. Stir for 20 seconds to toast the spices, adding a splash of water if the pan looks dry to prevent burning.
Pour in the coconut milk and tamarind paste, stirring to combine into a smooth sauce. Add the remaining 1/2 tsp salt and the slit green chili.
Bring the sauce to a gentle simmer over medium-low heat and cook for 5 minutes to let the flavors meld; do not let it boil hard, as this can cause the coconut milk to separate.
Gently slide the marinated fish chunks into the simmering sauce in a single layer, spooning some sauce over the top.
Cover and simmer on low heat for 6-8 minutes, without stirring too much to keep the fish pieces intact, until the fish is opaque throughout and reaches an internal temperature of 145F.
Taste the sauce and adjust salt or tamarind as needed for the right tangy balance.
Garnish with fresh cilantro and serve hot over steamed rice.