Heat oil in large non-stick skillet over medium heat. Add onion and garlic and cook, stirring frequently until onion is golden brown, about five minutes. Add tomatoes. Stir well and cook until tomatoes are just heated through, about 2 minutes. Add vinegar and stir well.
Push tomato mixture to one side of skillet. Add water. When liquid starts to simmer, add fish and cover the skillet. Cook til fish flakes easily, 8-10 minutes.
Remove fish and arrange on serving platter. Add pepper to taste to the tomato mixture and stir. Spoon the tomato mixture over the fish and serve immediately.Serves 4.