01 - Preheat oven to 375°F (190°C). 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 with cold water to stop cooking, and set aside.
03 - In a large mixing bowl, combine cooked chicken, ricotta, drained spinach, 1 cup mozzarella, 1/2 cup Parmesan, egg, garlic, salt, pepper, and Italian herbs. Mix thoroughly until evenly incorporated.
04 - Spread 1 cup of Alfredo sauce evenly across the bottom of the greased baking dish.
05 - Fill each cooked shell with approximately 2 tablespoons of the chicken-spinach filling. Arrange filled shells in the baking dish, opening side facing up, in a single layer.
06 - Pour the remaining Alfredo sauce over the stuffed shells, ensuring each shell receives even coverage.
07 - Sprinkle 1/2 cup mozzarella and 1/4 cup Parmesan cheese evenly over the sauce-covered shells.
08 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes at 375°F.
09 - Remove foil and continue baking for 10 additional minutes, or until cheese becomes golden brown and sauce bubbles around the edges.
10 - Remove from oven and let rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.