01 - Preheat the oven to 400°F. Lightly grease a baking dish or ovenproof skillet with cooking spray or a thin layer of olive oil.
02 - In a mixing bowl, combine the chopped spinach, softened cream cheese, shredded mozzarella, grated Parmesan, chopped sun-dried tomatoes, minced garlic, 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Stir until all ingredients are evenly incorporated.
03 - Using a sharp knife, cut a deep horizontal pocket into the side of each chicken breast, taking care not to slice all the way through to the opposite side.
04 - Divide the spinach and cheese filling evenly among the four chicken breasts, spooning it into each pocket. Secure the openings with toothpicks if needed to keep the filling enclosed.
05 - In a small bowl, mix together the Italian seasoning, paprika, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Rub this seasoning blend over all surfaces of each stuffed chicken breast.
06 - Heat 1 tablespoon of olive oil in a large ovenproof 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 the prepared baking dish and drizzle with the remaining 1 tablespoon of olive oil. Bake for 20 to 25 minutes until the internal temperature of the thickest part reaches 165°F.
08 - Remove the chicken from the oven and let it rest for 5 minutes. Remove any toothpicks before serving.