Preheat oven to 350°F. Grease 9x13 inch baking dish.
Cook shells until al dente. Drain. Rinse with cold water.
Mix chicken, ricotta, ½ cup mozzarella, Parmesan, egg, garlic, Italian seasoning, salt, pepper, and parsley.
For sauce, melt butter in saucepan over medium heat.
Add garlic. Cook 30 seconds.
Add heavy cream. Simmer gently.
Reduce to low. Whisk in Parmesan, salt, pepper, and nutmeg.
Stir until cheese melts and sauce thickens (3 to 4 minutes). Remove from heat.
Pour 1 cup Alfredo sauce in bottom of baking dish.
Stuff each shell with 2 tablespoons chicken filling.
Arrange shells in dish seam side up.
Pour remaining sauce over shells.
Sprinkle 1 cup mozzarella on top.
Cover with foil.
Bake 25 minutes covered.
Remove foil. Bake 5 more minutes until cheese is bubbly.
Rest 5 minutes.
Garnish with parsley. Serve hot.