01 - Set the oven temperature to 375°F (190°C).
02 - Bring a large pot of salted water to a boil. Cook the jumbo pasta shells until al dente according to package instructions, then drain and allow to cool slightly.
03 - In a medium bowl, combine shredded chicken, ricotta, 1 cup mozzarella, 0.25 cup Parmesan, egg, garlic powder, Italian herbs, salt, and black pepper. Mix thoroughly until well blended.
04 - Melt butter in a saucepan over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Stir in heavy cream and milk, bringing to a gentle simmer. Add 0.75 cup Parmesan and nutmeg if using, whisking until cheese melts and sauce thickens. Season with salt and pepper, then remove from heat.
05 - Spread 1 cup of Alfredo sauce evenly in the bottom of a 9x13-inch baking dish.
06 - Fill each pasta shell with approximately 2 tablespoons of the chicken mixture. Arrange the stuffed shells in the baking dish atop the sauce.
07 - Pour the remaining Alfredo sauce over the stuffed shells. Sprinkle with 0.5 cup shredded mozzarella and 2 tablespoons grated Parmesan cheese.
08 - Cover the dish with foil and bake in the preheated oven for 25 minutes.
09 - Remove the foil and continue baking for an additional 10 minutes, or until cheese is golden and bubbling.
10 - Sprinkle chopped fresh parsley over the baked shells before serving.