1. In a heavy 4-quart flameproof casserole, heat 1 tablespoon of annatto oil over moderate heat, tipping the casserole to spread the oil evenly over the bottom.
2. Spread half the onion slices, tomato and chili strips in the hot oil, and sprinkle them with half the garlic, oregano, salt and pepper. Lay the fish slices on top and cover them with the remaining onions, tomatoes, chilies and seasonings.
3. Sprinkle the remaining 2 tablespoons of annatto oil evenly over the top. Cover the casserole and simmer over low heat for 30 minutes, until the fish is opaque and firm. Do not overcook.
4. Serve either directly from the casserole, or with a spatula carefully transfer the fish and vegetables to a platter.
