Heat 1 tablespoon annatto oil in a heavy 4-quart pot over moderate heat, tipping pot to spread oil across the bottom.
Spread half the onion slices, tomatoes, and chili strips in the hot oil.
Sprinkle with half the garlic, oregano, salt and pepper.
Lay fish steaks on top of vegetable mixture.
Cover with remaining vegetables and seasonings.
Drizzle remaining annatto oil over top.
Cover and simmer over low heat until fish is opaque and firm, 20-30 minutes. Do not overcook.
NOTE: to make annatto oil, heat 1/2 cup vegetable oil in a saucepan over moderate heat. Stir in 1/4 cup annatto (achiote) seeds. Stir for 30 seconds. Reduce heat to low, cover and simmer for 10 minutes. Remove from heat, cool, and strain. This will keep several months if tightly covered and refrigerated, but it will lose flavor as it ages.