Preheat your grill to medium-high heat. While it heats, season both sides of the chicken with Italian seasoning, garlic powder, onion powder, salt, and pepper. This step locks in flavor before the cooking even starts.
Lightly oil the grill grates, then place the chicken on the grill. Cook each side for 6–7 minutes or until it reaches an internal temperature of 165°F. The key is to keep the heat steady so it cooks through without drying out.
When the chicken is nearly done, lay a slice of fresh mozzarella on top of each breast. Close the grill lid for about 2–3 minutes, just until the cheese melts and turns creamy.
While the chicken grills, mix the balsamic vinegar and brown sugar in a small saucepan. Simmer on low until it reduces by half and thickens. This sweet-tangy glaze will tie the whole dish together.
Place the grilled chicken on a serving plate, top each with a slice of tomato and fresh basil leaves, then drizzle generously with the balsamic reduction. Serve right away for the freshest taste.