Spray large nonstick skillet with cooking spray. Cook and stir onion over medium heat 5 minutes. Stir in tomato, capers and pepper. Cook 3 minutes.
Preheat broiler. Combine garlic and olive oil in a small bowl. Brush bread slices with oil mixture. Top with onion mixture, sprinkle with cheese. Place on baking sheet. Broil for 3 minutes or until cheese melts.