01 - Preheat oven to 375°F (190°C).
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 until well blended.
05 - Fill each chicken breast pocket generously with the spinach-cheese mixture. Secure the openings with toothpicks if necessary to keep filling intact.
06 - Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. Sear the stuffed chicken breasts for 2–3 minutes per side until golden brown.
07 - Transfer the skillet to the preheated oven (or move chicken to a baking dish). Bake for 22–25 minutes, or until the internal temperature reaches 165°F (74°C).
08 - Remove toothpicks, garnish with fresh basil or parsley if desired, and serve hot.