Season the fish with ½ teaspoon salt and set aside while you prepare the curry base.

In a large skillet, warm 2 tablespoons of cooking oil over medium heat.

Add the mustard seeds and cumin seeds to the hot oil, letting them sizzle for about 30 seconds until fragrant.

Add the chopped onion and sauté for 4-5 minutes, or until soft and golden.

Stir in the minced garlic, ginger, curry leaves, and slit red chilies. Cook for another 2 minutes until the mixture becomes aromatic.

Add the ground coriander, turmeric, and sugar, stirring well to combine the spices with the onion mixture.

Add the diced tomatoes and tomato paste to the mixture.

Stir well, allowing the tomatoes to cook down for about 8-10 minutes.

Stir occasionally until the sauce thickens and the oil begins to separate.

Gently add the seasoned fish into the curry sauce, making sure the fillets are well coated.

Lower the heat and cover the pan.

Let it simmer for 8 to 10 minutes, or until the fish is thoroughly cooked and can be easily flaked with a fork.

Garnish with extra fresh curry leaves or coriander if desired, and serve hot with steamed rice or naan.