01 - Preheat oven to 375°F. Position the rack in the center of the oven.
02 - Using a sharp knife, cut a deep pocket into the side of each chicken breast, taking care not to slice all the way through. Keep the edges intact to hold the filling securely.
03 - Season both sides of each chicken breast evenly with kosher salt, freshly ground black pepper, and garlic powder.
04 - In a mixing bowl, combine the softened cream cheese, shredded mozzarella, chopped spinach, sun-dried tomatoes, minced garlic, crushed red pepper flakes (if using), and grated Parmesan. Stir until all ingredients are evenly incorporated.
05 - Divide the filling evenly among the four chicken breasts, spooning it generously into each pocket. Secure the openings with toothpicks if needed to prevent the filling from escaping during cooking.
06 - Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. Once the oil is shimmering, 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 and the juices run clear.
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 if desired, and serve immediately while hot.