01 - Preheat oven to 375°F (190°C). Position the rack in the center of the oven.
02 - Using a sharp knife, carefully slice a horizontal pocket into the side of each chicken breast, stopping about ½ inch from the opposite edge to keep the pocket intact.
03 - Generously season both sides of each chicken breast with salt, black pepper, and garlic powder, ensuring even coverage.
04 - In a mixing bowl, combine the softened cream cheese, shredded mozzarella, chopped spinach, sun-dried tomatoes, minced garlic, crushed red pepper flakes, and Parmesan cheese. Stir until all ingredients are evenly incorporated.
05 - Divide the filling evenly among the four chicken breasts, spooning it into each pocket. Secure the openings with toothpicks if needed to keep the filling enclosed during cooking.
06 - Heat 1 tablespoon of olive oil in a large oven-safe skillet over medium-high heat. Once the oil shimmers, place the stuffed chicken breasts in the skillet and sear 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, or until the internal temperature of the thickest part reaches 165°F (74°C).
08 - Remove the chicken from the oven and discard any toothpicks. Let rest for 3 to 5 minutes. Garnish with fresh chopped basil or parsley if desired, and serve immediately while hot.