Sauté onion in hot oil in an ovenproof skillet or pan over medium heat 3 minutes or until tender. Add garlic, 1/4 teaspoon salt, and 1/4 teaspoon pepper; sauté 1 minute. Reduce heat to low; stir in wine, olives, and capers.
Sprinkle fish evenly with remaining salt and pepper. Place fish over onion mixture.
Bake, covered, at 400° for 26 minutes or until fish flakes with a fork. Sprinkle with diced tomatoes and parsley.