Preheat the Oven: Preheat the oven to 375°F (190°C). Lightly coat a 9 x 13-inch (23 x 33 cm) baking dish with nonstick cooking spray. This step ensures that the casserole does not stick to the dish, making it easier to serve once baked.

Cook the Pasta Shells: Bring a large pot of water to a rolling boil and add 1 tablespoon fine salt. Add the 24 jumbo shells and cook about 2 minutes less than the package directions, until flexible but still firmly al dente. This prevents the shells from becoming too soft during baking.

Drain and Cool the Shells: Drain the shells carefully and arrange them in a single layer on a clean baking sheet or large plate. Let them cool enough to handle. Select the 20 least-damaged shells for filling. Cooling them prevents burns and makes them easier to fill.

Prepare the Chicken Filling: In a large bowl, combine the cooked chicken, ricotta, 1 cup (115 g) mozzarella, 1/2 cup (45 g) Parmesan, spinach, egg, parsley, Italian seasoning, garlic powder, salt, and black pepper. Mixing these ingredients creates a flavorful filling that complements the pasta shells.

Check the Filling Texture: Stir until evenly combined: The mixture should be thick enough to mound on a spoon without running. If the ricotta was unusually wet, let the filling stand for a few minutes before stuffing the shells. This ensures the filling holds its shape inside the shells.

Start the Alfredo Sauce: Melt the butter in a 2- to 3-quart saucepan over medium-low heat. Add the garlic and cook for about 30 seconds, stirring constantly, until fragrant but not browned. Properly cooking the garlic enhances the flavor of the sauce without bitterness.

Add the Cream: Pour in the heavy cream and add the black pepper and nutmeg. Bring to a gentle simmer over medium-low heat and cook for 3 to 4 minutes, stirring frequently. This step helps to develop the flavors of the sauce before adding the cheese.

Add the Parmesan: Reduce the heat to low: Add the 1 1/2 cups (135 g) Parmesan gradually, whisking until smooth after each addition. The sauce should coat the back of a spoon but still pour easily. Avoid boiling the sauce after adding the cheese to prevent it from becoming grainy.

Sauce the Baking Dish: Spread about 1 1/2 cups (360 ml) of the Alfredo sauce evenly over the bottom of the prepared baking dish. This layer keeps the pasta moist during baking and adds flavor to the dish.

Fill the Shells: Spoon about 3 tablespoons of chicken filling into each of the 20 cooked shells. The shells should be generously filled but still hold their shape. Properly filling the shells ensures a balanced flavor in every bite.

Arrange the Stuffed Shells: Place the filled shells open-side up in the baking dish in a snug, even layer. This arrangement allows the sauce to cover the shells evenly, promoting even cooking.

Add the Remaining Sauce: Spoon the remaining Alfredo sauce evenly over and around the shells. Leave a few small areas of pasta exposed so they can develop lightly golden edges. This step enhances the texture and visual appeal of the dish.

Add the Cheese Topping: Sprinkle the remaining 1 cup (115 g) mozzarella and 1/4 cup (22 g) Parmesan evenly over the casserole. The cheese will melt and create a delicious, bubbly topping.

Bake Covered: Cover the baking dish loosely with aluminum foil and bake at 375°F (190°C) for 20 minutes. This helps to steam the shells and keep them moist during the initial baking period.

Brown the Top: Remove the foil and continue baking for 10 to 15 minutes, until the sauce is bubbling around the edges and the cheese is melted with lightly golden spots. This step creates a desirable texture contrast between the creamy filling and the crispy cheese topping.

Check the Temperature: Check the center of the casserole with an instant-read thermometer. Because the filling contains previously cooked chicken and egg, it should reach at least 165°F (74°C). This ensures that the dish is safe to eat.

Rest Before Serving: Remove the casserole from the oven and let it rest for 10 minutes. The sauce will thicken slightly and the shells will hold together more easily, making serving simpler.

Garnish and Serve: Sprinkle with optional chopped parsley and serve warm. This final touch adds a pop of color and freshness to the dish.