1. 1. Prepare the Pasta Shells: Bring a large pot of salted water to a rolling boil. Add the jumbo pasta shells and cook according to package directions until al dente. You want them tender but still firm enough to handle without tearing. Drain the shells well and rinse them briefly with cold water to prevent sticking, then set aside.

2. 2. Mix the Filling: In a large mixing bowl, combine the shredded or diced cooked chicken, ricotta cheese, 1/2 cup grated Parmesan cheese, the large egg, garlic powder, Italian seasoning, salt, and black pepper. Mix everything thoroughly until all ingredients are well combined and the filling is uniform.

3. 3. Preheat and Prepare Baking Dish: Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking dish with cooking spray or a little butter to prevent sticking.

4. 4. Assemble the Casserole: Spread about 1 cup of the Alfredo sauce evenly over the bottom of the prepared baking dish. Carefully take each cooked pasta shell and fill it generously with the chicken and cheese mixture. Arrange the filled shells in a single layer in the baking dish over the Alfredo sauce.

5. 5. Add More Sauce and Cheese: Once all the shells are filled and arranged, pour the remaining Alfredo sauce evenly over the stuffed shells, making sure to cover them as much as possible. Sprinkle the shredded mozzarella cheese generously over the top of the casserole.

6. 6. Bake to Golden Perfection: Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the casserole is bubbly around the edges and the mozzarella cheese on top is melted and lightly golden brown. If you like a crispier, more golden top, you can briefly broil it for 1-2 minutes at the very end, watching carefully to prevent burning.

7. 7. Rest and Serve: Once baked, remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the sauce to set slightly and makes serving easier. Garnish with fresh parsley or more Parmesan cheese if desired.