Rub skin of rock cod with half a lemon, sprinkle lightly with salt, and set aside while making sauce.
Combine onion, garlic, ginger, turmeric, cumin, and water in blender and purée to a smooth paste. Heat oil, add purée and cook, stirring constantly, for about 5 minutes. Add tomato paste, hot pepper, and cilantro and continue cooking and stirring until well integrated, then remove the curry paste from heat. Season with 1/2 teaspoon salt, the pepper, and the lemon juice.
Heat the oven to 400°F. Butter a piece of foil large enough to envelope the fish to within 1.5 inches of the foil's edges. Wipe fish, cut 4 diagonal slashes on each side, and brush evenly with curry paste, inside and out. Wrap fish in foil and bake for 30 minutes. Loosen foil to expose fish and place under broiler to Brown and finish cooking, about 10 minutes. Based while broiling.
Transfer fish to a platter and serve with pan juices, rice, and condiments, such as: chopped cucumbers and tomatoes, roasted peanuts, coconut shreds, chutney, sliced bananas, raisins, yogurt, and chopped green peppers.
Ling cod, striped bass, red snapper, redfish, or cod may also be used for this recipe.