01 - Preheat oven to 400°F. Grease a baking dish with a little olive oil.
02 - Pat chicken breasts dry. With a sharp knife, cut a horizontal pocket into each breast, being careful not to cut all the way through.
03 - In a bowl, combine ricotta, spinach, garlic, Parmesan, Italian herbs, nutmeg, egg yolk, salt, and pepper. Mix until smooth.
04 - Stuff each chicken breast with the ricotta-spinach mixture, securing with toothpicks if needed.
05 - Rub chicken breasts with olive oil, then season both sides with salt and pepper.
06 - Place stuffed chicken in prepared baking dish. Top each breast with mozzarella and extra Parmesan.
07 - Bake for 25–30 minutes, or until chicken is cooked through (internal temperature 165°F) and cheese is golden.
08 - Let rest 5 minutes before serving. Remove toothpicks, slice, and serve hot.