Preheat the oven to 350°F. Cook the shells according to the package directions, then drain and rinse under cold water until cool enough to handle.
In a medium bowl, stir the ricotta together with 1/2 cup of the parmesan, the egg, Italian seasoning, salt, pepper, and 1 cup of the mozzarella until evenly combined. Fold in the cooked chicken.
Coat a 9x13 baking dish with nonstick spray. Spread about 1/2 cup of the Alfredo sauce across the bottom.
Fill each shell with a heaping spoonful of the chicken and cheese mixture, roughly 24 shells total, and arrange them in the dish.
Pour the remaining Alfredo sauce evenly over the stuffed shells.
Scatter the remaining 1/2 cup mozzarella and 1/4 cup parmesan across the top.
Cover the dish loosely with foil and bake for 20 minutes.
Remove the foil and bake another 10 minutes, until the cheese has melted and the sauce is bubbling at the edges.
Let stand 5 to 10 minutes before serving.