01 - Set the oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente as per package directions. Drain and set aside.
03 - Combine shredded chicken, chopped spinach, ricotta, mozzarella, Parmesan, egg, minced garlic, Italian herbs, salt, and pepper in a large bowl. Mix thoroughly.
04 - Melt butter in a saucepan over medium heat. Add garlic and sauté for 1 minute until aromatic. Stir in heavy cream and bring to a gentle simmer.
05 - Whisk in Parmesan, salt, pepper, and nutmeg if using. Cook while stirring until sauce thickens slightly, about 3 to 4 minutes. Remove from heat.
06 - Spread 1 cup of Alfredo sauce evenly on the bottom of the greased baking dish.
07 - Fill each pasta shell with the chicken and spinach mixture. Arrange stuffed shells in the baking dish.
08 - Pour the remaining Alfredo sauce evenly over the arranged shells. Sprinkle shredded mozzarella and grated Parmesan over the top.
09 - Cover the dish with foil and bake for 20 minutes.
10 - Remove foil and continue baking for an additional 10 to 15 minutes until the cheese is bubbly and golden.
11 - Allow to cool for 5 minutes. Garnish with chopped fresh parsley before serving if desired.