Preheat the Oven: Set your oven to 375°F (190°C). Lightly spray a 9×13-inch baking dish with nonstick spray and set aside.

Cook the Pasta Shells: Bring a large pot of water to a boil. Cook the jumbo pasta shells according to the package instructions until al dente. Drain the shells and place them upside down on a baking sheet to cool and dry.

Prepare the Filling: In a large mixing bowl, combine the lightly beaten egg, ricotta cheese, 1 cup of shredded mozzarella cheese, ¼ cup of shredded Parmesan cheese, sour cream, Italian seasoning, salt, and pepper. Mix until well combined, then fold in the shredded chicken.

Stuff the Shells: Using a spoon, fill each cooked pasta shell with the cheese and chicken mixture. Arrange the stuffed shells side by side, open side up, in the prepared baking dish.

Add the Sauce and Cheese: Pour the Alfredo sauce evenly over the stuffed shells. Sprinkle the remaining ½ cup of mozzarella cheese and ½ cup of Parmesan cheese on top.

Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.

Garnish and Serve: Let the dish cool for a few minutes. Garnish with chopped fresh parsley before serving.