Rinse and pat dry the fish fillets, sprinkle with salt and turmeric, and let sit for 10 minutes.
Heat oil in a pan and lightly fry the fish for 2 minutes per side, then remove and set aside.
In the same pan, sauté chopped onions until soft and golden.
Add garlic and ginger paste, cooking for 1 minute until aromatic.
Stir in mustard paste, mustard seeds, turmeric, chili powder, and a splash of water to make a smooth sauce.
Pour in coconut milk or cream and simmer for 2–3 minutes.
Gently place the fish into the sauce and simmer for 5 minutes until fully cooked.
Garnish with green chili and fresh coriander, adjust salt to taste, and serve hot.