01 - Preheat oven to 375°F. Position the rack in the center of the oven.
02 - Using a sharp knife, slice a pocket into the side of each chicken breast, being careful not to cut all the way through. Keep the incision neat so the filling stays enclosed during cooking.
03 - Season both sides of each chicken breast evenly with salt, black pepper, and garlic powder.
04 - In a mixing bowl, combine the 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 all ingredients are evenly distributed.
05 - Divide the spinach-cheese filling among the four chicken breasts, spooning it generously into each pocket. Secure the openings with toothpicks if needed to keep the filling contained.
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 into the preheated oven, or move the chicken to a baking dish. Bake for 22 to 25 minutes until the internal temperature of the thickest part reaches 165°F.
08 - Remove from the oven and discard any toothpicks. Let the chicken rest for 3 to 5 minutes. Garnish with freshly chopped basil or parsley and serve hot.