Stir together the ingredients for the marinade in a small bowl.
Butterfly the chicken breasts and place them in a large ziplock back with the dressing. Toss well and marinate in the refrigerator for at least 2 hours.
Grill the chicken on high heat for about 5 minutes per side or until cooked through.
Layer mozzarella and tomato slices on top and cook on low for 2-3 minutes more until the cheese is melted.
Top with fresh basil and drizzle with balsamic if desired.