01 - Set the oven to 400°F (200°C) to preheat.
02 - Carefully create a horizontal pocket in each chicken breast using a sharp knife, avoiding cutting through completely.
03 - Season both inside and outside of each chicken breast with salt and freshly ground black pepper.
04 - Insert slices of mozzarella, tomato, and 4 basil leaves into each chicken breast pocket; secure with toothpicks as necessary.
05 - Heat olive oil in an oven-safe skillet over medium-high heat and sear stuffed chicken breasts for 2 to 3 minutes per side until golden brown.
06 - Place the skillet in the preheated oven and bake for 20 to 25 minutes until cooked through and cheese is melted.
07 - Combine balsamic vinegar and honey in a small saucepan, simmer over medium heat for 5 to 8 minutes, stirring occasionally, until reduced to a syrupy consistency; remove from heat.
08 - Remove toothpicks from the chicken breasts, drizzle the balsamic glaze over them, and garnish with additional basil leaves if desired.