For the sauce, heat half the oil over medium heat in a sauce pan. Add the seed spices; after they sizzle and begin to pop, about 1 min, add onions and garlic. Cook, stirring, until onions are slightly browned. Stir in ¾ t salt, ¼ t cayenne, cumin and curry powder and cook for 1 min until fragrant. Add the tomatoes, chopped cilantro and water. Bring to a boil, reduce heat and simmer 10 min until slightly thickened. (Sauce may be made ahead and refrigerated for up to 5 days.)
For the halibut, combine ½ t salt, ¼ t cayenne and ½ t turmeric. With paper towels, pat halibut fillets dry and rub the fillets on both sides with the salt mixture. Set aside for 30 min.
Meanwhile, preheat the oven to 350°. Heat remaining oil in sauté pan and sear the fish on both sides until lightly browned. Transfer to a baking dish and pour sauce over the fish. Bake, uncovered, for 10 to 15 min until fish is done. Garnish with cilantro sprigs.
Serve with rice, dal, naan, chutney, etc.