01 - Preheat oven to 400°F. Lightly grease a baking dish with cooking spray or a small amount of olive oil.
02 - Place each chicken breast on a cutting board. Using a sharp knife, slice horizontally through the thickest part of each breast, opening it like a book without cutting completely through. Pound gently to even thickness if needed.
03 - Season both sides of each butterflied chicken breast generously with 1/2 teaspoon salt and 1/2 teaspoon black pepper, ensuring even coverage.
04 - In a medium mixing bowl, combine ricotta cheese, chopped spinach, 1/3 cup parmesan, mozzarella, minced garlic, nutmeg, 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Mix thoroughly until all ingredients are evenly incorporated.
05 - Divide the spinach ricotta mixture evenly among the four chicken breasts, spreading it over one half of each butterflied piece. Fold the opposite side over the filling to enclose completely.
06 - Secure the edges of each stuffed breast with toothpicks if necessary to prevent filling from escaping during baking. Place chicken pieces in the prepared baking dish, seam-side down.
07 - Brush the tops of each chicken breast with 1 tablespoon olive oil. Sprinkle the remaining 1/4 cup grated parmesan cheese evenly over all pieces.
08 - Bake for 25 to 30 minutes, or until chicken reaches an internal temperature of 165°F when measured with a meat thermometer, and the parmesan topping is golden brown.
09 - Remove from oven and let chicken rest for 5 minutes to allow juices to redistribute. Carefully remove toothpicks before slicing crosswise and serving warm.