Preheat the oven to 400°F (200°C) or heat a grill to medium-high. Rub the chicken breasts with olive oil, garlic powder, Italian seasoning, salt, and pepper. Grill or bake the chicken for 18–20 minutes, or until the internal temperature reaches 165°F (74°C).
In a small bowl, combine diced tomatoes, red onion, basil, balsamic vinegar, olive oil, salt, and pepper. Mix well and let sit while the chicken cooks to allow flavors to meld.
Once the chicken is cooked, place on a baking sheet (if grilled) or leave in the baking dish (if baked). Spread about 1 tablespoon of pesto over each chicken breast, then top with a slice of mozzarella.
Place the chicken under the broiler for 2–3 minutes until the cheese is melted and bubbly.
Spoon the bruschetta topping over each chicken breast. Garnish with fresh basil if desired, and serve immediately.