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 completely through.
03 - Combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg if using, egg yolk, salt, and pepper in a mixing bowl. Mix until thoroughly blended and smooth.
04 - Divide filling evenly among chicken breasts, stuffing each pocket. Secure openings with toothpicks if necessary to prevent filling from escaping during baking.
05 - Rub chicken breasts with olive oil. Season both sides generously with salt and pepper. Arrange in prepared baking dish.
06 - Sprinkle mozzarella and additional Parmesan evenly over the top of each stuffed chicken breast.
07 - Bake for 25 to 30 minutes until chicken reaches internal temperature of 165°F and cheese is melted and golden brown.
08 - Let chicken rest for 5 minutes before serving. Remove toothpicks, slice crosswise to reveal filling, and serve immediately.