01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil or cooking spray.
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.
03 - Season the chicken breasts generously with salt and black pepper on both sides.
04 - In a mixing bowl, combine ricotta cheese, chopped spinach, 1/4 cup grated Parmesan, minced garlic, dried Italian herbs, nutmeg (if using), salt, and black pepper. Stir until well blended.
05 - Spoon the spinach-ricotta mixture evenly into the pocket of each chicken breast. Secure the openings with toothpicks if needed to hold the filling in place.
06 - Heat olive oil in a large skillet over medium-high heat. Sear the stuffed chicken breasts for approximately 2 minutes per side until lightly golden brown.
07 - Transfer the seared chicken breasts to the prepared baking dish. Sprinkle shredded mozzarella and remaining 2 tablespoons grated Parmesan evenly over each breast.
08 - Bake for 20 to 25 minutes, or until the chicken is cooked through with an internal temperature of 165°F and the cheese is melted and bubbling.
09 - Remove toothpicks before serving. Allow the chicken to rest for 5 minutes, then serve hot.