Step 1: Prepare the Marinade

In a small bowl, whisk together the olive oil, balsamic vinegar, Dijon mustard, Italian seasoning, garlic, salt, and black pepper until well combined.

Place the chicken breasts into a resealable bag or shallow dish and pour the marinade over the chicken.

Coat each breast evenly and refrigerate for at least 1 hour or up to 24 hours.

Discard the marinade before baking.

Step 2: Prepare the Chicken

Preheat the oven to 400°F (200°C).

Line a rimmed baking sheet with foil or parchment paper and lightly coat it with nonstick cooking spray.

Place each chicken breast on a cutting board.

Using a sharp knife, cut 4 deep slices across each chicken breast, spacing them about ½ inch apart without cutting all the way through.

Step 3: Stuff the Chicken

Place one slice of mozzarella, one slice of tomato, and half of a basil leaf into each cut.

Continue filling every pocket until all the chicken breasts are stuffed.

Arrange the stuffed chicken on the prepared baking sheet.

Step 4: Bake

Bake for 25–30 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).

The mozzarella should be melted and the tomatoes softened.

Step 5: Finish and Serve

Allow the chicken to rest for 5 minutes before serving.

Drizzle with balsamic glaze if desired.

Garnish with freshly chopped basil and an extra drizzle of balsamic vinegar.

Serve immediately.