01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil to prevent sticking.
02 - Pat chicken breasts dry with paper towels. Using a sharp knife, cut a horizontal pocket into the side of each breast, taking care not to slice completely through to the other side.
03 - Combine ricotta, chopped spinach, minced garlic, grated Parmesan, Italian herbs, nutmeg (if using), egg yolk, salt, and pepper in a mixing bowl. Mix until fully incorporated and smooth.
04 - Fill each pocket with the ricotta-spinach mixture, dividing evenly among the four breasts. Secure openings with toothpicks if necessary to keep filling contained during cooking.
05 - Rub the exterior of each breast with olive oil. Season both sides generously with salt and pepper.
06 - Arrange stuffed chicken in the prepared baking dish. Sprinkle mozzarella and additional Parmesan evenly over the tops of the breasts.
07 - Bake for 25–30 minutes until chicken reaches an internal temperature of 165°F and cheese is melted and golden brown.
08 - Let chicken rest for 5 minutes before serving to allow juices to redistribute. Remove any toothpicks, slice crosswise to reveal the filling, and serve immediately.