Preheat the oven to 375 degrees Fahrenheit.
Pat the chicken breasts dry. Lay a chicken breast flat on a cutting board. Using a sharp knife, carefully cut a 3-4 inch wide pocket into the thickest part of the breast, being sure not to split the breast completely. Repeat with each breast, then season on both sides with salt and pepper to taste.
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add onion and garlic and saute for 3-5 minutes, stirring often, until softened.
Stir in spinach and parsley, and cook for 2-3 minutes, until spinach wilts and moisture evaporates. Transfer to a bowl and allow to cool completely. Clean out the skillet and set it aside.
Meanwhile, in a large bowl, mix together the ricotta, mozzarella cheese, Parmesan cheese, egg yolk, oregano, nutmeg, salt and pepper. When the spinach mixture has cooled, stir it into the cheese mixture until combined.
Divide the stuffing mixture into the pocket of each chicken breast.
In the clean skillet, heat remaining olive oil over medium-high heat until hot. Add the chicken in and cook on one side for 5-7 minutes until golden, then carefully turn over and cook on the other side for another 5 minutes. Place the skillet in the oven and bake for 16-20 minutes, or until the internal temperature in the thickest part of the chicken is 165 degrees F.
Transfer the chicken to a serving platter and allow it to rest for 3-5 minutes before serving. Serve with a salad or your favorite vegetables and starches.