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 the meat.
03 - Combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg if using, egg yolk, salt, and pepper in a mixing bowl. Mix until fully incorporated and smooth.
04 - Fill each chicken breast pocket with an equal portion of the ricotta-spinach mixture. Secure openings with toothpicks if necessary to prevent filling from escaping during baking.
05 - Rub olive oil evenly over each chicken breast. Season both sides generously with salt and pepper.
06 - Place stuffed chicken breasts in the prepared baking dish. Top each breast with shredded mozzarella and additional Parmesan cheese.
07 - Bake for 25-30 minutes until chicken is completely cooked through, reaching an internal temperature of 165°F, and cheese topping is golden brown and bubbly.
08 - Remove from oven and let chicken rest for 5 minutes before serving. Remove toothpicks, slice crosswise to reveal filling, and serve immediately.