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 without cutting completely through.
03 - Combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg, egg yolk, salt, and pepper in a bowl. Mix until thoroughly incorporated and smooth.
04 - Fill each chicken breast pocket with ricotta-spinach mixture. Secure openings with toothpicks if necessary to prevent filling from escaping.
05 - Rub olive oil over exterior of each breast. Season both sides generously with salt and pepper.
06 - Arrange stuffed chicken in prepared baking dish. Sprinkle mozzarella and additional Parmesan evenly over each breast.
07 - Bake for 25-30 minutes until internal temperature reaches 165°F and cheese is golden and bubbly.
08 - Allow chicken to rest for 5 minutes before serving. Remove toothpicks, slice crosswise to reveal filling, and serve immediately.