Preheat oven to 375°F. Lightly spray a 9×13-inch baking dish with nonstick spray. Set aside.: The oven warmth should feel steady when you open it and the air should be noticeably warm, not scorching. This temperature allows the filling to heat through and the cheeses to melt gently, producing a creamy interior rather than a dried out top. A common mistake is skipping preheating, which leads to uneven baking and longer cook time, so always wait until the oven registers the correct temperature. If your dish sticks, the nonstick spray will prevent cheese from bonding to the pan.
Bring a pot of water on the stovetop to a boil. Cook the pasta shells to al dente. After cooking the pasta shells, drain and place them upside down on a baking sheet to dry.: As the water reaches a rolling boil, you will hear lively bubbling and see steam rising; add a generous pinch of salt for seasoning. Cook until the shells are tender but still firm at the center, they should give slightly under pressure. Draining and placing them upside down stops residual steam from making them soggy, and you want them just dry enough to hold the filling. Overcooking will cause tearing during stuffing, so test one shell before draining all. If shells stick together, separate gently with a fork to avoid ripping.
Make the filling. In a large bowl, combine the egg, ricotta cheese, 1 cup mozzarella cheese, 1/4 cup parmesan cheese, sour cream, Italian seasoning, salt, and pepper. Mix well, then fold in the shredded chicken.: When you mix, the bowl will carry a tangy, cheesy scent and the texture should become smooth and slightly glossy, indicating the egg and sour cream have emulsified with the cheeses. Folding in the shredded chicken distributes savory pockets throughout the mixture, giving balanced bites. Avoid overmixing, which can break down the ricotta and make the filling runny. If the filling feels too loose, chilling for 10 minutes firms it slightly and helps with stuffing.
Stuff each shell with the filling and arrange them side by side, open side up, in the prepared baking dish.: When filling, you will notice the shells cradling the mixture; press gently so the filling nests without tearing the edges. Arrange them snugly so they support one another and do not tip over as the cheeses melt. A frequent issue is overfilling, which leads to spilling and uneven baking, so use a spoonful per shell and keep the top level with the rim. If a shell rips, replace it with a reserve shell to maintain presentation.
Evenly spread the Alfredo sauce on top of the stuffed shells. Top with the remaining mozzarella and parmesan cheeses.: As you pour the Alfredo sauce , watch it pool and cascade into gaps, coating the shells and creating a glossy surface. The sauce keeps the shells moist and creates a cohesive casserole as it warms. Sprinkle the remaining cheeses evenly so the top melts uniformly and achieves golden color. A mistake is piling too much sauce in one area, which can lead to soggy pockets; aim for an even layer so baking is consistent.
Cover with foil and bake for 25 minutes, then remove the foil and continue baking for another 10 minutes, until bubbly. For golden cheese, broil for 1-3 minutes.: With the foil on, steam builds gently and the filling heats through evenly, you will hear a soft bubbling as moisture moves. Removing the foil allows the top to brown; look for bubbling around the edges and a lightly golden surface as cues it is ready. If you choose to broil, watch closely because broiling goes from perfect to burnt in seconds; stand by the oven and listen for the sizzling and watch the color change. A common error is leaving the foil on too long, which prevents browning, so time the transitions carefully.
Garnish with fresh parsley and serve.: The final garnish adds a fresh scent and bright green contrast to the creamy casserole, lightening each bite visually and on the palate. Serve hot so the cheese pulls and the sauce remains silky. If you let it cool too long, the texture firms and it will be less luscious, so plan to serve shortly after resting five to ten minutes. Use a spatula to lift shells carefully so they keep their shape on the plate.