Preheat oven to 425°.
Place oil in a large nonstick skillet over medium-high heat until hot. Add onion and next 3 ingredients; cook 10 minutes or until vegetables are crisp-tender, stirring frequently. Add tomato and salt; cook 2 minutes, stirring frequently.
Place fillets in a 13- x 9-inch baking dish coated with cooking spray. Place onion mixture on top of fillets. Sprinkle with olives. Bake, uncovered, at 425° for 20 minutes or until fish flakes easily when tested with a fork.