Preheat oven to 425°.
Place fish in an 11 x 7-inch baking dish coated with cooking spray; drizzle with 1 tablespoon oil, and sprinkle with salt and pepper. Bake at 425° for 20 minutes or until fish flakes easily when tested with a fork.
While fish bakes, heat remaining 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add onion, mushrooms, and garlic; sauté 5 minutes or until tender. Add tomato, olives, and capers. Cook, uncovered, 3 minutes or until thoroughly heated, stirring frequently. Spoon mixture over cooked fish, and sprinkle with basil.