01 - Preheat oven to 375°F. Position rack in the center of the oven.
02 - Using a sharp knife, cut a deep pocket into the side of each chicken breast, being careful not to slice all the way through. Keep the opening as small as possible while creating enough room for the filling.
03 - Season both sides of each chicken breast evenly with salt, 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, sun-dried tomatoes, minced garlic, red pepper flakes, and Parmesan. Stir until all ingredients are evenly distributed.
05 - Divide the filling mixture 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 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. Bake for 22 to 25 minutes until the internal temperature of the thickest part of each breast reaches 165°F when checked with an instant-read thermometer.
08 - Remove from the oven and carefully discard any toothpicks. Let the chicken rest for 3 to 5 minutes before slicing. Garnish with freshly chopped basil or parsley and serve immediately.