Thai Fish in Red Sauce

1. Heat oil and fry onions over moderate heat until soft and golden brown.
2. Add tomatoes, vinegar, salt and pepper to taste, and the chilies.
3. Cover and simmer for 20 minutes or until tomatoes are pulpy and the sauce thick.
4. Add fish, cover and cook until fish is done.
5. Serve hot, sprinkled with fresh coriander leaves.