In a tall jug or milkshake container, combine the curry paste ingredients. Use an immersion blender to blitz on high for 5 to 10 seconds until the onion is pureed. Add more water if needed for a smoother consistency.

Heat the vegetable oil in a large pot over medium heat. Once hot, add the black mustard seeds, allowing them to sizzle for about 30 seconds.

Add the chopped red onion to the pot and sauté for approximately 3 minutes, until the edges begin to turn golden.

Add the prepared curry paste, cooking for about 3 minutes to evaporate excess water and allow the spices to bloom.

Raise the heat to medium-high, then add the tomato paste and tomato pulp. Cook for an additional 2 minutes.

Pour in the 2/3 cup of water, the coconut milk, sugar, salt, and optional chili powder. Stir, then bring the mixture to a gentle simmer over low heat.

Add the tomato wedges and green chilies, allowing another 3 minutes for the sauce to thicken slightly.

Carefully stir in the fish cubes and cook for 3 to 4 minutes, or until the fish is opaque and flakes easily with a fork.

Transfer the curry to a serving bowl, garnish with fresh coriander and optional green chilies, and serve hot with basmati rice.