01 - Preheat oven to 400°F. Grease a baking dish with a small amount of olive oil.
02 - Pat chicken breasts dry with paper towels. Using a sharp knife, cut a horizontal pocket into each breast, being careful not to cut completely through the meat.
03 - Combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg, egg yolk, salt, and pepper in a bowl. Mix until fully incorporated and smooth.
04 - Fill each chicken breast pocket with the ricotta-spinach mixture. Secure openings with toothpicks if necessary to prevent filling from escaping.
05 - Rub the exterior of each stuffed breast with olive oil. Season both sides generously with salt and pepper.
06 - Arrange chicken in the prepared baking dish. Sprinkle mozzarella and additional Parmesan evenly over the tops of each breast.
07 - Bake for 25-30 minutes until chicken reaches an internal temperature of 165°F and cheese is golden and bubbly.
08 - Let chicken rest for 5 minutes before serving. Remove toothpicks, slice crosswise to reveal filling, and serve hot.