01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente according to package directions. Drain and set aside.
03 - In a large bowl, combine shredded chicken, chopped spinach, ricotta, mozzarella, Parmesan, egg, garlic, Italian herbs, salt, and pepper. Stir until thoroughly mixed.
04 - Melt butter in a saucepan over medium heat. Add garlic and sauté for 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer.
05 - Whisk in Parmesan, salt, pepper, and nutmeg if using. Cook, stirring, until sauce thickens slightly, about 3 to 4 minutes. Remove from heat.
06 - Spread 1 cup of Alfredo sauce evenly in the bottom of the prepared baking dish.
07 - Stuff each cooked shell with the filling mixture and arrange them in the baking dish.
08 - Pour remaining Alfredo sauce evenly over the filled shells. Sprinkle with mozzarella and Parmesan cheeses.
09 - Cover with aluminum foil and bake for 20 minutes.
10 - Remove foil and continue baking for 10 to 15 minutes until the top is bubbly and golden.
11 - Allow to cool for 5 minutes before garnishing with chopped parsley, if desired, and serving.