01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente. Drain and set aside.
03 - In a large bowl, mix cooked chicken, ricotta, chopped spinach, Parmesan, egg, garlic powder, salt, and pepper until well combined.
04 - Spread 1 cup of Alfredo sauce evenly over the bottom of the greased baking dish.
05 - Fill each cooked shell with approximately 2 tablespoons of the chicken-spinach mixture and arrange them in the baking dish.
06 - Pour the remaining Alfredo sauce over the arranged stuffed shells.
07 - Evenly sprinkle shredded mozzarella and remaining Parmesan cheese over the sauce-covered shells.
08 - Cover the dish with aluminum foil and bake for 25 minutes.
09 - Remove foil and continue baking for 10 minutes until cheese is golden and bubbly.
10 - Allow to rest 5 minutes before serving. Garnish with chopped fresh parsley if desired.