01 - Preheat oven to 375°F and lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente, then drain and set aside.
03 - In a large bowl, mix shredded chicken, chopped spinach, ricotta, mozzarella, Parmesan, egg, garlic, Italian herbs, salt, and pepper until thoroughly blended.
04 - Melt butter in a saucepan over medium heat. Sauté minced garlic for 1 minute until fragrant. Stir in heavy cream and bring to a gentle simmer.
05 - Whisk in Parmesan cheese, salt, pepper, and nutmeg if using. Cook, stirring, until slightly thickened, approximately 3-4 minutes, then remove from heat.
06 - Spread 1 cup of Alfredo sauce on the bottom of the prepared baking dish. Fill each pasta shell with the chicken-spinach mixture and arrange in the dish.
07 - Pour remaining Alfredo sauce evenly over the stuffed shells. Sprinkle with mozzarella and Parmesan cheese topping.
08 - Cover dish with foil and bake for 20 minutes. Remove foil and bake an additional 10-15 minutes until bubbly and golden brown.
09 - Allow to rest for 5 minutes before serving. Garnish with fresh parsley if desired.