In a large skillet, heat the olive oil over medium heat. Add the onion and garlic; cook until the onion is tender, 2 to 3 minutes. Add the tomatoes, mushrooms, olives, oregano, salt and pepper. Bring the sauce to boiling.
Gently place the fish in the sauce in the skillet, spooning the sauce over the fish. Return to boiling. Reduce the heat; simmer, covered, for 8 to 10 minutes or until fish flakes easily when tested with a fork. With a wide spatula, lift the fish from the skillet and move it to a serving dish. Spoon the sauce over the fish.
Notes
Serve the fish with the rice and/or crusty bread slices.