01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with oil or butter.
02 - Boil salted water and cook jumbo pasta shells until al dente. Drain and set aside.
03 - In a large bowl, combine cooked chicken, ricotta cheese, chopped spinach, Parmesan, egg, garlic powder, salt, and black pepper. Mix thoroughly until evenly blended.
04 - Spread 1 cup of Alfredo sauce evenly across the bottom of the prepared baking dish.
05 - Fill each cooked shell with approximately 2 tablespoons of the filling mixture. Arrange the stuffed shells in the baking dish over the sauce.
06 - Pour the remaining Alfredo sauce evenly over the arranged stuffed shells.
07 - Sprinkle shredded mozzarella and remaining Parmesan evenly over the top of the shells.
08 - Cover the baking dish with aluminum foil and bake for 25 minutes.
09 - Remove the foil and bake for an additional 10 minutes, until the cheese is golden and bubbly.
10 - Allow the dish to rest for 5 minutes before serving. Garnish with fresh parsley if desired.