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, black pepper, and garlic powder.
04 - In a mixing bowl, combine softened cream cheese, shredded mozzarella, chopped spinach, drained and chopped sun-dried tomatoes, minced garlic, crushed red pepper flakes, and grated Parmesan. Mix thoroughly until well blended.
05 - Fill each chicken breast pocket generously with the spinach-cheese mixture. Secure the openings with toothpicks if needed to keep the filling contained.
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 a golden-brown crust forms.
07 - Transfer the skillet directly to the preheated oven (or move the chicken to a baking dish). Bake for 22 to 25 minutes until the internal temperature reaches 165°F.
08 - Remove from oven, discard toothpicks, and let rest briefly. Garnish with fresh chopped basil or parsley if desired and serve hot.