01 - Preheat oven to 375°F.
02 - Using a sharp knife, slice a pocket into the side of each chicken breast, being careful not to cut all the way through.
03 - Season both sides of the chicken breasts evenly with salt, pepper, and garlic powder.
04 - In a mixing bowl, combine cream cheese, mozzarella, spinach, sun-dried tomatoes, minced garlic, crushed red pepper flakes (if using), and Parmesan cheese. Mix thoroughly until well blended.
05 - Fill each chicken breast pocket generously with the spinach-cheese mixture. Secure the openings with toothpicks if necessary to prevent filling from escaping.
06 - Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. Sear the stuffed chicken breasts for 2 to 3 minutes per side until golden brown.
07 - Transfer the skillet to the preheated oven or move the chicken to a baking dish. Bake for 22 to 25 minutes until the chicken is cooked through and reaches an internal temperature of 165°F.
08 - Remove toothpicks, garnish with fresh basil or parsley if desired, and serve hot.