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 each breast, opening the side without cutting completely through.
03 - In a mixing bowl, combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg, egg yolk, salt, and pepper. Mix thoroughly until smooth and evenly incorporated.
04 - Fill each chicken pocket with the ricotta-spinach mixture. Secure openings with toothpicks to prevent filling from escaping during cooking.
05 - Rub exterior of chicken breasts with olive oil. Season both sides generously with salt and pepper.
06 - Place stuffed chicken in prepared baking dish. Sprinkle mozzarella and additional Parmesan evenly over each breast.
07 - Bake for 25-30 minutes until chicken reaches internal temperature of 165°F and cheese topping is golden brown and bubbly.
08 - Remove from oven and let chicken rest for 5 minutes. Remove toothpicks carefully. Slice crosswise to display filling and serve immediately.