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 thickest side of each chicken breast, taking care not to slice all the way through. Open the pocket gently to create ample space for the filling.
03 - Season both sides of each chicken breast evenly with kosher salt, freshly ground black pepper, and garlic powder. Set aside while preparing the filling.
04 - In a mixing bowl, combine the softened cream cheese, shredded mozzarella, chopped spinach, drained sun-dried tomatoes, minced garlic, crushed red pepper flakes if using, and grated Parmesan. Stir until all ingredients are thoroughly incorporated.
05 - Divide the filling evenly among the four chicken breasts, spooning it generously into each pocket. Press the edges together to enclose the filling and secure with toothpicks if needed to prevent leaking during cooking.
06 - Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat until the oil shimmers. Sear the stuffed chicken breasts for 2 to 3 minutes per side until a deep 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 registers 165°F when measured at the thickest part of the breast.
08 - Remove from the oven and carefully extract any toothpicks. Let the chicken rest for 3 to 5 minutes before slicing. Garnish with freshly chopped basil or parsley and serve immediately.