01 - Set oven temperature to 400°F (200°C).
02 - Heat 1 tablespoon olive oil in a medium skillet over medium heat. Add minced garlic and cook for 30 seconds. Add spinach and sauté until wilted, about 2 minutes. Remove from heat and allow to cool slightly.
03 - Combine sautéed spinach, crumbled feta, chopped sun-dried tomatoes (if used), parsley, dried oregano, and ground black pepper in a bowl. Mix thoroughly.
04 - Pat chicken breasts dry. Using a sharp knife, create a deep pocket along the side of each breast without cutting through completely.
05 - Sprinkle salt and black pepper evenly over the outside surfaces of each chicken breast.
06 - Fill each pocket with the spinach and feta mixture, securing openings with toothpicks if needed.
07 - Arrange stuffed chicken breasts in a baking dish. Drizzle 1 tablespoon olive oil over them.
08 - Place baking dish in the oven and cook for 25 to 30 minutes, or until the internal temperature reaches 165°F (74°C) and juices run clear.
09 - Allow chicken to rest for 5 minutes before removing toothpicks and serving.