01 - Set the oven temperature to 375°F and lightly grease a 9x13-inch baking dish.
02 - Boil the jumbo pasta shells in salted water until al dente, then drain and set aside.
03 - In a large bowl, combine the shredded chicken, ricotta cheese, chopped spinach, grated Parmesan, egg, garlic powder, salt, and black pepper; mix thoroughly.
04 - Spread 1 cup of Alfredo sauce evenly over the bottom of the prepared baking dish.
05 - Fill each cooked shell with approximately 2 tablespoons of the filling and arrange them in the baking dish.
06 - Pour the remaining Alfredo sauce over the stuffed shells.
07 - Sprinkle shredded mozzarella and remaining Parmesan cheese evenly over the assembled dish.
08 - Cover the dish with aluminum foil and bake for 25 minutes.
09 - Remove the foil and continue baking for 10 minutes more, until cheese is golden and bubbly.
10 - Let the dish rest for 5 minutes before serving; garnish with chopped fresh parsley if desired.