Spray bottom of large nonstick skillet with vegetable oil spray. Place onion, 2 tbsp wine and garlic in skillet. Cover skillet; cook over medium-low heat until onion is tender, stirring occasionally, about 10 minutes. Add tomatoes, 1 teaspoon oregano and 6 tbsp wine; simmer uncovered until mixture is reduce to thick sauce, about 10 minutes. Place fish on sauce. Sprinkle fish with 1/2 tsp oregano, salt and pepper. Cover skillet; simmer until fish is just opaque in center, about 6 minutes.
Spoon sauce onto plates, dividing equally. Place fish atop sauce on serve.