Place the fish on a plate and rub over with the curry powder and salt. Heat 3 tablespoons of oil in a large non-stick frying pan until hot. Lightly dust fish pieces with flour and add to the oil. Cook until fish is lightly browned, about 1 minute, turning once. Do not fully cook fish. Remove fish to the plate. To the same pan add onions, garlic, red pepper, mustard, and curry leaves; cook until the onions are soft, about 3 minutes. Stir in the water, tomatoes, and tamarind and bring to a boil. Simmer until the sauce is reduced and thickened, about 8 minutes. Gently slip in the fish and the accumulated juices. Cook until the fish is heated through, about 2 minutes. Serve garnished with herbs.
Combine coriander, fenugreek, mustard, cumin, black pepper, fennel, cinnamon, cloves, and grind to a fine powder using an electric grinder. Stir in the turmeric and red pepper and transfer into a jar. Cover tightly and keep in a cool dry place.
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