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, stopping before cutting all the way through.
03 - In a medium bowl, combine ricotta, chopped spinach, minced garlic, grated Parmesan, Italian herbs, nutmeg, egg yolk, salt, and pepper. Mix thoroughly until completely smooth and evenly incorporated.
04 - Divide filling evenly among chicken breasts, stuffing each pocket completely. Secure openings with toothpicks if needed to prevent filling from escaping during cooking.
05 - Rub olive oil over the exterior of each stuffed breast. Season both sides generously with salt and pepper.
06 - Arrange stuffed chicken in prepared baking dish. Sprinkle mozzarella and additional Parmesan evenly over the top of each breast.
07 - Bake for 25 to 30 minutes until chicken reaches internal temperature of 165°F and cheese topping is bubbly and golden brown.
08 - Remove from oven and let chicken rest for 5 minutes to allow juices to redistribute. Remove toothpicks, slice crosswise to reveal filling, and serve immediately.