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1
Pour the juice of 1 lemon onto the fish fillets and sprinkle with 1/2 teaspoon of the salt and 1/4 teaspoon of the turmeric; set aside.
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2
In a spice grinder, combine the mustard seeds, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon turmeric, 1 chile (or to taste) and the garlic.
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3
Grind to a smooth paste.
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4
Transfer to a food processor and add 3 tablespoons of the mustard oil, 2 tablespoons lemon juice, 1/3 cup of the cilantro and 3 tablespoons water.
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5
Process until smooth, adding more water if needed to get a paste with the consistency of cake batter.
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6
In a large skillet over medium heat, heat 2 tablespoons mustard oil and add the spice paste.
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7
Let it sizzle for a few moments, and then use a wooden spatula to flip and stir it in the oil, reducing the heat if it starts to brown too quickly.
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8
Saute until it darkens and thickens, about 10 minutes.
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9
Scoop the paste out of the pan and into a small bowl.
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10
Wipe out the frying pan with a paper towel and return the pan to medium-high heat.
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11
Heat remaining 1 1/2 tablespoons mustard oil.
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12
When the oil is shimmering, add the nigella seeds and, if desired, a whole green chile.
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13
Fry for a minute or two, then add the fish filets, carefully flipping them to brown both sides.
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14
When the fish is browned, add the spice paste and 1/4 cup hot water or as needed to loosen the paste.
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15
Bring it to a boil and simmer until the fish is cooked through, about 5 minutes depending on the thickness of the fillets.
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16
Sprinkle with chopped cilantro.
