Preheat oven to 350°F (175°C). Cook jumbo pasta shells according to package directions until al dente. Drain and set aside to cool for about 5 minutes.

In a large mixing bowl, combine shredded chicken, ricotta cheese, 1 cup mozzarella, Parmesan, egg, garlic powder, parsley, basil, salt, and pepper. Mix until well combined and the mixture holds together when scooped.

Melt butter in a large skillet over medium heat. Add minced garlic and sauté for 1-2 minutes until fragrant but not browned. Pour in heavy cream and bring to a gentle simmer.

Gradually add Parmesan cheese to the cream, whisking constantly until melted and sauce thickens to coat the back of a spoon, about 3-4 minutes. Add black pepper and nutmeg if using. Sauce should have a smooth, velvety consistency.

Spread 1 cup of Alfredo sauce on the bottom of a 9x13-inch baking dish. Stuff each pasta shell with a heaping 2 tablespoons of chicken mixture and place in the dish, open side up.

Pour remaining Alfredo sauce over the stuffed shells, ensuring they are well-coated. Sprinkle remaining 1/2 cup mozzarella cheese on top.

Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 10 minutes until cheese is golden brown with bubbling edges. Let cool for 5 minutes before serving.