Marinate the fish fillets with 1 teaspoon salt and ½ teaspoon turmeric. Keep aside for 15 minutes.
Thinly slice the onion and tomato. Slice the cherry tomatoes in half.
Place a pan on medium heat and add the coconut oil. Once the oil heats up, add the fennel and black mustard seeds. Allow it to sizzle for a few seconds and then, add the ginger garlic, onion plus few curry leaves. Sauté for 1 minute.
Sprinkle ½ teaspoon salt and ½ teaspoon turmeric. Mix it around.
In a bowl, add the coriander powder, Kashmiri chili powder and fennel powder along with 1-tablespoon water. Give it a mix and pour it to the pan. Mix it all around and then, drop halved cherry tomatoes (save a little bit to be added at the end). Sauté for 5 minutes or until onion and tomatoes are cooked through and softened.
Pour 1 and ½ cup coconut milk along with the turmeric and tomato paste. Stir around and add the green chili along with ½ cup water.
Layer the fish fillets in one layer, put rest of the halved cherry tomatoes and few more curry leaves. Cook the curry for 5 to 7 minutes depending on the size of fish fillets.
Check for salt and add any, if needed.
Serve warm!