Preheat the oven to 350°F. Cook the jumbo pasta shells according to package directions until al dente, about 9 to 12 minutes. They should be tender but still have a slight firmness. Drain carefully and lay on a baking sheet to cool for about 5 minutes.

In a large mixing bowl, combine the shredded chicken, ricotta cheese, 1 cup of mozzarella, Parmesan, egg, garlic powder, parsley, basil, salt, and pepper. Mix until well blended. The mixture should be creamy and hold together on a spoon.

Melt the butter in a large skillet over medium heat. Add the minced garlic and cook for 1 to 2 minutes until fragrant but not browned. Watch closely to prevent burning.

Stir in the heavy cream and bring to a gentle simmer with small bubbles forming around the edges. Gradually whisk in the Parmesan cheese, stirring constantly until the sauce thickens and becomes smooth, about 3 to 4 minutes. The sauce should coat the back of a spoon. Season with black pepper and nutmeg if using.

Lightly grease or spray a 9x13-inch baking dish. Spread 1 cup of the Alfredo sauce across the bottom. Stuff each cooled pasta shell with about 2 tablespoons of the chicken mixture. Arrange the stuffed shells in the dish with the open side up, nestling them close together.

Pour the remaining Alfredo sauce over the stuffed shells, making sure each one is well coated. Use the back of a spoon to spread sauce into any gaps. Sprinkle the remaining 1/2 cup of mozzarella cheese on top.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is melted, bubbly, and lightly golden on top. Let rest for 5 minutes before serving.