01 - Preheat oven to 375°F. Position the rack in the center of the oven.
02 - Using a sharp knife, cut a horizontal pocket into the side of each chicken breast, taking care not to slice all the way through. Open the pocket gently to create even space for the filling.
03 - Season both sides of each chicken breast evenly with salt, black pepper, and garlic powder, pressing the spices gently into the meat.
04 - In a mixing bowl, combine the softened cream cheese, shredded mozzarella, chopped spinach, sun-dried tomatoes, minced garlic, red pepper flakes, and Parmesan cheese. Stir until all ingredients are evenly distributed.
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 is shimmering, 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 to the preheated oven (or move the chicken to a baking dish). Bake for 22 to 25 minutes until the internal temperature reaches 165°F at the thickest part of the breast.
08 - Remove from the oven and discard any toothpicks. Let the chicken rest for 3 to 5 minutes. Garnish with fresh chopped basil or parsley if desired and serve immediately.