01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil.
02 - Pat chicken breasts dry with paper towels. Using a sharp knife, cut a horizontal pocket into the center of each breast, taking care not to cut completely through to the other side.
03 - In a mixing bowl, combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg if using, egg yolk, salt, and pepper. Mix until fully incorporated and smooth.
04 - Spoon the ricotta-spinach mixture into each chicken pocket, dividing evenly. Secure openings with toothpicks if necessary to prevent filling from escaping during cooking.
05 - Rub the exterior of each stuffed breast with olive oil. Season both sides generously with salt and pepper. Place chicken in the prepared baking dish, spaced evenly apart.
06 - Sprinkle shredded mozzarella and additional grated Parmesan over the top of each chicken breast.
07 - Bake for 25 to 30 minutes until chicken reaches an internal temperature of 165°F and cheese is melted and lightly golden.
08 - Remove from oven and let chicken rest for 5 minutes. Remove toothpicks carefully. Slice crosswise to display the filling and serve immediately.