Remove bones from the fish fillets and cut into portions. Sprinkle with 0.5 teaspoon turmeric and 0.5 teaspoon salt, massage into the fish, and let rest for 10 minutes.
Use a hand blender to blend 1 onion, 8 garlic cloves, 1 tomato, roughly 1-2 fresh green chilli peppers (or to taste), 1 teaspoon mustard seeds, 0.5 teaspoon salt, 0.5 teaspoon turmeric, and 0.5 teaspoon cumin into a smooth paste.
Heat 2 tablespoons mustard oil over high heat until it just begins to smoke. Fry the fish pieces for 1-2 minutes on each side until golden. Remove with a slotted spoon and drain on paper towels.
In the same pot, cook the spice paste covered over medium heat for 2-3 minutes. Uncover and continue cooking until the oil begins to separate from the paste. Return the fish pieces to the pot and gently turn them several times to coat evenly with the paste.
Add 300 ml hot water and stir gently. Cover the pot, reduce heat to low, and simmer for 5-10 minutes.