01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or nonstick cooking spray.
02 - Place chicken breasts on a clean cutting board. Using a sharp knife, carefully slice a horizontal pocket into the side of each breast, stopping about 1/2 inch from the edges to avoid cutting all the way through.
03 - Season the outside of each chicken breast generously with salt and freshly ground black pepper on both sides.
04 - In a medium mixing bowl, combine the ricotta cheese, chopped spinach, 1/4 cup Parmesan cheese, minced garlic, dried Italian herbs, nutmeg if using, and a pinch of salt and pepper. Stir until all ingredients are evenly incorporated.
05 - Divide the filling evenly among the four chicken breasts, spooning it into each pocket. Use toothpicks to secure the openings if needed to keep the filling contained during cooking.
06 - Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the stuffed chicken breasts for approximately 2 minutes per side until a light golden crust forms.
07 - Transfer the seared chicken breasts to the prepared baking dish. Sprinkle the shredded mozzarella and remaining 2 tablespoons of grated Parmesan evenly over the top of each breast.
08 - Bake in the preheated oven for 20 to 25 minutes, or until the internal temperature of the chicken reaches 165°F when measured with a meat thermometer and the cheese is melted and bubbling.
09 - 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.