01 - Preheat oven to 400°F. Lightly grease a baking dish.
02 - Heat 1 tablespoon olive oil in a skillet over medium heat. Add chopped spinach and cook until wilted, about 2 to 3 minutes. Remove from heat and allow to cool slightly.
03 - In a mixing bowl, combine cooked spinach, cream cheese, shredded mozzarella, grated Parmesan, minced garlic, nutmeg, salt, and pepper. Stir until fully blended.
04 - Pat the chicken breasts dry. Using a sharp knife, carefully cut a horizontal slit along the side of each breast to form a pocket without cutting through.
05 - Season both the interior and exterior of the chicken breasts with salt and ground black pepper.
06 - Fill each chicken pocket evenly with the spinach and cheese mixture. Secure openings with toothpicks if needed.
07 - Place the stuffed breasts in the prepared baking dish and drizzle with 1 tablespoon olive oil.
08 - Bake in the preheated oven for 25 to 30 minutes, or until the internal temperature reaches 165°F and juices run clear.
09 - Allow the chicken to rest for 5 minutes before serving. Remove toothpicks prior to serving.