01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
02 - Cook the jumbo pasta shells in salted boiling water according to package directions until al dente. Drain, rinse under cold water to stop cooking, and set aside.
03 - In a large bowl, combine the cooked chicken, ricotta cheese, spinach, 1 cup mozzarella, 1/4 cup Parmesan, egg, garlic, Italian herbs, salt, and pepper. Mix until thoroughly incorporated.
04 - Spread 1 cup of Alfredo sauce evenly in the bottom of the prepared baking dish to create a foundation layer.
05 - Fill each cooked pasta shell with approximately 2 tablespoons of the chicken-spinach mixture. Arrange the stuffed shells in the baking dish, open side up, in a single layer.
06 - Pour the remaining Alfredo sauce evenly over the stuffed shells, covering them completely.
07 - Sprinkle the remaining 1 cup mozzarella and 1/4 cup Parmesan cheese evenly over the top of the shells.
08 - Cover the baking dish with aluminum foil and bake for 25 minutes.
09 - Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden brown.
10 - Let the dish rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.