01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente. Drain and set aside.
03 - In a large bowl, mix shredded chicken, chopped spinach, ricotta, mozzarella, Parmesan, egg, garlic, dried herbs, salt, and pepper until well blended.
04 - Melt butter in a saucepan over medium heat. Sauté garlic for 1 minute until aromatic. Add heavy cream and simmer gently.
05 - Whisk in Parmesan, salt, pepper, and nutmeg if desired. Cook, stirring, for 3–4 minutes until slightly thickened. Remove from heat.
06 - Spread 1 cup Alfredo sauce on baking dish base. Fill each pasta shell with filling mixture and arrange in dish.
07 - Pour remaining Alfredo sauce over stuffed shells. Sprinkle with mozzarella and Parmesan cheeses.
08 - Cover with foil and bake for 20 minutes.
09 - Remove foil and bake an additional 10–15 minutes until bubbly and golden.
10 - Let rest for 5 minutes. Garnish with chopped parsley before serving if desired.