Flatten chicken breasts:
Take a chicken breast and set it on a piece of parchment; fold the paper over and flatten with a rolling pin until it's just under 1 inch thick. Do the same for the other breasts.Mix seasoning blend:
Mix Italian seasoning, sea salt, garlic powder, onion powder, and pepper in a small bowl; stir until well combined.Season chicken with oil and spices:
Coat the chicken with olive oil, then apply the seasoning mix evenly on both sides.Grill chicken until cooked:
Preheat a grill or grill pan to high heat. Cook the chicken for 5-6 minutes on each side, until an instant-read thermometer reads 155°F (70°C).Add mozzarella and melt:
Place 2 slices of fresh mozzarella on each breast. Continue grilling for 3 minutes, until the cheese is melted and the internal temperature reaches 165°F (75°C).Top with tomatoes and basil:
Move the chicken to a serving platter or individual plates. Add 2-3 tomato slices on top, sprinkle with fresh basil, and season with salt and pepper as desired. Right before serving, drizzle with aged balsamic vinegar or balsamic glaze. Serve immediately.