1-Start by making the balsamic reduction first: Start by making the balsamic reduction first since it needs time to simmer and cool slightly before serving. Pour the balsamic vinegar and brown sugar into a small saucepan over medium heat, stirring occasionally until the mixture reduces by half and becomes syrupy, about 10 minutes-it should coat the back of a spoon. While the reduction simmers, pat the chicken breasts dry and combine the Italian seasoning, garlic powder, onion powder, salt, and pepper in a small bowl. Rub the oil over both sides of the chicken, then coat generously with the seasoning mixture, making sure each piece is evenly covered for consistent flavor.
2-Heat your grill to medium-high heat: Heat your grill to medium-high heat. Once the grill is hot and the seasoning has adhered to the chicken, place the breasts directly on the grates. Grill for 6-7 minutes per side until the chicken reaches an internal temperature of 160°F (the chicken will still be slightly undercooked at this point). I like to let the chicken develop a nice char on the first side before flipping, which adds great flavor and visual appeal to the final dish.
3-Once the chicken is nearly cooked through,: Once the chicken is nearly cooked through, quickly top each breast with a slice of fresh mozzarella. Close the grill lid and cook for an additional 2-3 minutes until the cheese is melted and creamy, and the chicken reaches a final internal temperature of 165°F. Watch carefully during this stage so the cheese melts evenly without the chicken overcooking.
4-Transfer the grilled chicken to a serving: Transfer the grilled chicken to a serving platter or individual plates. Top each breast with a tomato slice and a generous handful of fresh basil ribbons. For the best presentation and flavor, I arrange the tomato and basil while the chicken is still warm so they soften slightly from the heat. Drizzle the balsamic reduction over each piece just before serving, allowing it to pool slightly on the plate for a restaurant-quality presentation.