Heat a nonstick grill pan over medium-high heat. Sprinkle fish with salt and pepper. Coat pan with cooking spray. Add fish to pan; cook 5 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, heat oil in a large nonstick skillet over medium heat. Add onion; cook 4 minutes or until tender, stirring frequently. Add garlic, oregano, and tomatoes; bring to a boil. Reduce heat, and simmer 6 minutes, stirring frequently. Stir in olives and capers; cook 1 minute. Spoon tomato mixture over fish. Sprinkle with chopped parsley, if desired.