This recipe, brought to us by cookbook author Maya Kaimal, is exceptionally flavorful because the bluefish is marinated with a piquant rub of cayenne, turmeric, and salt before cooking.
Remove the head and tail from the fish. Butterfly by slicing it down the entire length of the belly side. Open it up, and carefully remove the backbone, starting from the neck. Remove any remaining bones. Fold fish in half, skin-side out, and rub skin with a mixture of 1/4 teaspoon cayenne, 1/4 teaspoon turmeric, and 1/4 teaspoon salt. Marinate for 15 minutes.
In a small skillet, heat ghee over medium heat; add cashews, and saute until golden. Remove to a plate. In the same skillet with remaining ghee, saute raisins until they swell and brown slightly. Add raisins to the plate with cashews, and set aside.
In a large skillet, heat 1/3 cup oil, and fry onions until edges are nicely browned. Add garlic, ginger, and green chiles, and stir for 1 minute. Stir in coriander, remaining 1/2 teaspoon cayenne, cumin, remaining 1/2 teaspoon turmeric, garam masala, tomato, lime juice, cilantro, mint, remaining teaspoon salt, and sugar, and fry until onions are well browned and mixture is thick and pastelike, adding enough water to keep it moist. Remove from heat, and cool slightly.
Open fish, and spread one-third of the onion mixture inside; fold it over to close. In the same skillet, push the remaining onion mixture to the sides of the pan. Add remaining 1 tablespoon oil, and place fish in the middle. Cook, covered, on medium-low heat, until bottom of fish is lightly browned, about 10 minutes. Carefully turn, and continue to cook until lightly browned, about 10 minutes more. Remove from heat.
Carefully transfer fish to a platter. Spread remaining sauce over fish. Sprinkle with cashews and raisins, and garnish with cilantro and mint; serve immediately.