Heat coconut oil in a pot. Add mustard seeds and fenugreek. Let them sizzle.
Add sliced onion, curry leaves, garlic, ginger, and green chili. Sauté until soft and golden.
Stir in turmeric, chili powder, and chopped tomato. Cook until the tomato breaks down.
Add the fish pieces and gently coat them in the spice mixture.
Pour in the coconut milk and a splash of water if needed. Add salt.
Simmer uncovered on low heat for 10–15 minutes, or until the fish is cooked through and the curry is thick and fragrant.
Finish with a squeeze of lime juice.
Serve hot with rice, roti, or string hoppers.