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.

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.

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.

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.

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.

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.

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.

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. Do not let it boil after adding the cheese.

Spread about 1 1/2 cups (360 ml) of the Alfredo sauce evenly over the bottom of the prepared baking dish.

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.

Place the filled shells open-side up in the baking dish in a snug, even layer.

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.

Sprinkle the remaining 1 cup (115 g) mozzarella and 1/4 cup (22 g) Parmesan evenly over the casserole.

Cover the baking dish loosely with aluminum foil and bake at 375°F (190°C) for 20 minutes.

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.

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).

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.

Sprinkle with optional chopped parsley and serve warm.