01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil or cooking spray and set aside.
02 - Place chicken breasts on a cutting board. Using a sharp knife, carefully slice a pocket into the side of each breast, taking care not to cut all the way through. Season both sides of each breast generously with salt and black pepper.
03 - In a mixing bowl, combine ricotta cheese, chopped spinach, 1/4 cup grated Parmesan, minced garlic, dried Italian herbs, nutmeg if using, and a pinch each of salt and pepper. Stir until all ingredients are evenly incorporated.
04 - Spoon the spinach-ricotta mixture evenly into the pocket of each chicken breast, dividing it equally among the four portions. Secure the openings with toothpicks if needed to keep the filling enclosed.
05 - Heat olive oil in a skillet over medium-high heat. Once the oil is shimmering, sear the stuffed chicken breasts for approximately 2 minutes per side until lightly golden on the exterior.
06 - Transfer the seared chicken breasts to the prepared baking dish. Sprinkle shredded mozzarella and remaining 2 tablespoons of grated Parmesan evenly over the top of each breast.
07 - Bake in the preheated oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the cheese topping is melted and bubbling.
08 - Remove from the oven and discard any toothpicks. Allow the chicken to rest for 5 minutes before serving to let the juices redistribute. Serve hot.