Season both sides of the chicken breasts with Italian seasoning, garlic powder, onion powder, salt, and black pepper.
Lightly coat the grill grates with nonstick cooking spray or another grill-safe cooking oil.
Place the chicken on the grill and cook until nearly cooked through, turning as needed for even cooking.
Top each chicken breast with one slice of mozzarella. Close the grill and cook for another 2–3 minutes, or until the cheese melts and the chicken reaches an internal temperature of 165°F (74°C).
Meanwhile, prepare the balsamic reduction. Add the balsamic vinegar and brown sugar to a small saucepan.
Bring to a gentle simmer and cook for about 10 minutes, stirring occasionally, until the mixture has reduced by approximately half and lightly coats the back of a spoon.
Remove the Chicken Caprese from the grill and top each breast with a fresh tomato slice and sliced basil.
Drizzle with the warm balsamic reduction and serve immediately.
Notes
Fresh mozzarella is ideal for a creamy finish, but low-moisture mozzarella can also be used.