Prepare the Marinade: Whisk together olive oil, balsamic vinegar, Dijon mustard, Italian seasoning, minced garlic, salt, and pepper in a small bowl. Place the chicken breasts in a resealable plastic bag or shallow dish and pour the marinade over them, ensuring all pieces are coated well. Seal the bag or cover the dish and refrigerate for at least 1 hour or up to 24 hours.
Preheat the Oven: Preheat your oven to 400℉ (200℃). Prepare your baking sheet by spraying it with non-stick cooking spray.
Prepare the Chicken Breasts: Make four deep cuts across the top of each chicken breast about ½ inch apart, ensuring you do not cut all the way through to create pockets for stuffing.
Stuff the Chicken: Fill each cut pocket in the chicken breasts with alternating slices of Roma tomato, fresh mozzarella, and basil leaves.
Bake the Chicken: Transfer each stuffed chicken breast onto your prepared baking pan and bake for approximately 25-30 minutes or until an instant-read thermometer reads 165℉ (75℃).
Serve and Enjoy: Once baked, let the chicken rest for about 5 minutes before serving. Drizzle with extra balsamic glaze if desired and garnish with additional chopped basil leaves.