01 - Set the oven temperature to 375°F (190°C).
02 - Bring a large pot of salted water to a boil and cook jumbo pasta shells until al dente according to package directions. Drain and let cool slightly.
03 - In a medium bowl, combine shredded chicken, ricotta, 1 cup mozzarella, 1/4 cup Parmesan, egg, garlic powder, dried Italian herbs, salt, and black pepper. Mix until fully incorporated.
04 - Melt unsalted butter in a saucepan over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Stir in heavy cream and milk, bringing the mixture to a gentle simmer. Whisk in Parmesan cheese and nutmeg if using, cooking until sauce thickens slightly. Season with salt and pepper, then remove from heat.
05 - Pour 1 cup of Alfredo sauce evenly into the bottom of a 9x13-inch baking dish. Stuff each pasta shell with approximately 2 tablespoons of the chicken filling and arrange them in the dish.
06 - Pour the remaining Alfredo sauce over the stuffed shells. Sprinkle 1/2 cup shredded mozzarella and 2 tablespoons grated Parmesan evenly on top.
07 - Cover the dish with foil and bake for 25 minutes in the preheated oven.
08 - Remove the foil and continue baking for an additional 10 minutes until the cheese is melted, bubbly, and golden.
09 - Sprinkle chopped fresh parsley over the baked shells before serving.