01 - Preheat oven to 375°F (190°C). Position the rack in the center.
02 - Using a sharp knife, slice a pocket into the side of each chicken breast, being careful not to cut all the way through to the other side.
03 - Season both sides of each chicken breast evenly with salt, pepper, and garlic powder.
04 - In a mixing bowl, combine the softened cream cheese, shredded mozzarella, chopped spinach, chopped sun-dried tomatoes, minced garlic, crushed red pepper flakes, and grated Parmesan. Mix thoroughly until well blended.
05 - Generously fill each chicken breast pocket with the spinach-cheese mixture. Secure the openings with toothpicks if needed to keep the filling enclosed.
06 - Heat 1 tablespoon of 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 (74°C).
08 - Remove the chicken from the oven and discard any toothpicks. Let rest for 5 minutes. Garnish with fresh chopped basil or parsley if desired, and serve hot.