Clean and cut the fish into chunks. Set aside.
Heat vegetable oil in a large pot over medium heat.
Add the chopped onion, minced garlic, and sliced green chilies. Sauté until the onion is translucent.
Add the chopped tomato, turmeric powder, and ground black pepper. Cook until the tomato is softened and the oil begins to separate.
Add the fish chunks to the pot. Cook for about 5-7 minutes, stirring occasionally, until the fish starts to cook through.
Pour in the coconut milk and fish sauce (or soy sauce). Stir well and bring to a simmer.
Reduce the heat and let it cook for about 10-15 minutes, or until the fish is fully cooked and the flavors are well combined.
Add lemon or lime juice to the pot. Adjust seasoning with salt as needed.
Garnish with fresh coriander if desired.
Enjoy your meal!