Prepare the Oven: Set your oven to 375°F (190°C) to preheat. Meanwhile, lightly coat a baking dish with grease or line it with parchment paper for easy cleanup.

Prepare the Filling: In a medium bowl, combine the chopped roasted red peppers, fresh spinach, shredded mozzarella cheese, and minced garlic. Flavor the mixture by adding Italian seasoning, along with salt and pepper, adjusting to your preference. Stir until well mixed.

Prepare the Chicken: Using a sharp knife, carefully slice a pocket into each chicken breast, being careful not to cut all the way through.

Stuff the Chicken: Spoon the roasted red pepper, spinach, and mozzarella mixture into the pockets of each chicken breast, pressing gently to seal the filling inside.

Sear the Chicken: In a large oven-safe skillet, heat the olive oil over medium heat. Place the stuffed chicken breasts into the skillet and sear for 2 to 3 minutes per side, until they achieve a golden brown exterior.

Bake the Chicken: If using balsamic vinegar, drizzle it over the chicken breasts. Transfer the skillet (or baking dish) to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (75°C).

Garnish and Serve: Remove the chicken from the oven and let it rest for a few minutes. Garnish with fresh basil, if desired, and serve hot with your favorite sides like roasted vegetables, potatoes, or a simple salad.