In a large skillet, heat olive oil over medium heat. Add onion and garlic; cook until onion is tender, for 2 to 3 minutes. Add tomatoes, mushrooms, olives, oregano, salt and pepper. Bring sauce to boiling.
Gently place fish in sauce in skillet, spooning sauce over fish. Return to boiling. Reduce heat; simmer, covered, for 8 to 10 minutes or until fish flakes easily when tested with a fork. With a wide spatula, lift fish from skillet to a serving dish. Spoon sauce over fish.
Serve with rice and/or crusty bread slices.
Nutritional Information per serving: (serving size: 6 ounces of fish + topping) Calories: 335, Fat: 16.5g, Saturated Fat: 2.41g, Sugar: 1g, Fiber: 3.5g, Protein: 35g, Cholesterol: 102mg, Carbohydrates: 12g