01 - Preheat oven to 375°F (190°C). 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, being careful not to slice all the way through. Keep the opening just wide enough to fill.
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 softened cream cheese, shredded mozzarella, chopped spinach, chopped sun-dried tomatoes, minced garlic, crushed red pepper flakes (if using), and grated Parmesan. Stir until all ingredients are evenly incorporated.
05 - Generously fill each chicken breast pocket with the spinach-cheese mixture. 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. 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 reaches 165°F (74°C).
08 - Remove from the oven and carefully remove any toothpicks. Let the chicken rest for 3 to 5 minutes, garnish with fresh chopped basil or parsley if desired, and serve hot.