01 - Preheat oven to 400°F. Lightly grease a baking dish or sheet pan with cooking spray or a small amount of olive oil.
02 - Pat chicken breasts dry with paper towels. Rub each breast with olive oil, then season evenly with salt, black pepper, and Italian seasoning on both sides. Place seasoned chicken in the prepared baking dish.
03 - Spoon 1 tablespoon of basil pesto over each chicken breast. Using the back of the spoon, spread the pesto evenly to coat the top surface of each piece.
04 - Place baking dish in the preheated oven. Bake chicken for 15 minutes until the pesto is fragrant and chicken begins to cook through.
05 - While chicken bakes, combine diced tomatoes, red onion, minced garlic, chopped fresh basil, extra-virgin olive oil, balsamic vinegar, 1/2 teaspoon salt, and 1/4 teaspoon black pepper in a medium mixing bowl. Toss thoroughly to combine and set aside to allow flavors to meld.
06 - Remove chicken from oven after 15 minutes. Top each breast with one-quarter of the bruschetta mixture, spooning evenly over the pesto layer. Sprinkle shredded mozzarella cheese over the top of each breast.
07 - Return dish to oven and bake for an additional 8 to 10 minutes. Cook until chicken reaches an internal temperature of 165°F and cheese is fully melted and lightly bubbly.
08 - Remove from oven and let chicken rest for 3 to 5 minutes before serving. This allows juices to redistribute. Serve with any remaining bruschetta mixture spooned alongside or over the top.