Heat coconut oil in a large pan or kadhai over medium heat. Add mustard seeds and let them splutter.
Add sliced onions, green chilies, ginger, garlic, and curry leaves. Sauté until the onions turn translucent.
Add chopped tomatoes and cook until they soften and turn mushy.
Stir in turmeric powder, red chili powder, coriander powder, and fenugreek seeds (if using). Cook for a minute until the spices are aromatic.
Pour in thick coconut milk and bring the curry to a gentle simmer.
Add the marinated fish pieces to the curry, ensuring they are submerged in the liquid.
Cover and cook for 8-10 minutes or until the fish is cooked through and tender.
Season the curry with salt according to taste. Garnish with fresh cilantro leaves.