01 - Preheat oven to 400°F. Lightly grease a baking dish.
02 - Heat 1 tablespoon olive oil in a skillet over medium heat. Add spinach and cook until wilted, about 2 to 3 minutes. Remove from heat and let cool slightly.
03 - In a mixing bowl, combine cooked spinach, cream cheese, mozzarella, Parmesan, garlic, nutmeg, salt, and pepper. Stir until fully incorporated.
04 - Pat chicken breasts dry. Using a sharp knife, cut a horizontal pocket along the side of each breast, taking care not to cut through completely.
05 - Season each chicken breast inside and out with salt and ground black pepper.
06 - Fill each pocket evenly with the spinach and cheese filling. Secure with toothpicks if needed.
07 - Place stuffed breasts in the prepared baking dish. Drizzle with 1 tablespoon olive oil. Bake for 25 to 30 minutes until chicken reaches an internal temperature of 165°F and juices run clear.
08 - Allow chicken to rest for 5 minutes before serving. Remove toothpicks prior to plating.