Preheat oven to 350 degrees. Lay the fish in a baking pan. Heat oil to a moderate temperature and fry the onion slices until they are transparent. Arrange over the fish. Combine the remaining ingredients in a blender or food processor until smooth. (Or crush chilies and garlic with a mortar and pestle and mash tomatoes well with a fork before combining with other ingredients.) Pour over fish, cover the pot, and simmer about 30 minutes until fish is just cooked.