03In a large skillet over medium heat, heat 2 tablespoons mustard oil and add the spice paste. 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. Sauté until it darkens and thickens, about 10 minutes. Scoop the paste out of the pan and into a small bowl.04Wipe out the frying pan with a paper towel and return the pan to medium-high heat. Heat remaining 1 1/2 tablespoons mustard oil. When the oil is shimmering, add the nigella seeds and, if desired, a whole green chile. Fry for a minute or two, then add the fish filets, carefully flipping them to brown both sides. When the fish is browned, add the spice paste and 1/4 cup hot water or as needed to loosen the paste. Bring it to a boil and simmer until the fish is cooked through, about 5 minutes depending on the thickness of the fillets. Sprinkle with chopped cilantro.