01 - Preheat oven to 375°F and lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Boil pasta shells according to package directions until al dente. Drain thoroughly, rinse under cold water to stop cooking, and set aside.
03 - In a large mixing bowl, combine shredded chicken, ricotta, drained spinach, mozzarella, Parmesan, egg, minced garlic, salt, pepper, and Italian herbs. Mix until evenly distributed.
04 - Spread 1 cup of Alfredo sauce across the bottom of the prepared baking dish in an even layer.
05 - Fill each cooked shell with approximately 2 tablespoons of the chicken-spinach filling. Place filled shells in the baking dish, opening facing up, arranging them in a single layer.
06 - Pour the remaining Alfredo sauce over the stuffed shells, ensuring each shell receives coverage.
07 - Sprinkle the additional mozzarella and Parmesan cheese evenly over the top of the dish.
08 - Cover the baking dish tightly with aluminum foil and bake for 25 minutes.
09 - Remove the foil and continue baking for 10 minutes, or until the cheese is golden brown and bubbly.
10 - Let the stuffed shells rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.