Preheat oven to 375°F (190°C).
Cook jumbo pasta shells in a large pot of salted boiling water until al dente. Drain and rinse with cool water to prevent sticking.
In a large mixing bowl, combine the shredded chicken, ricotta cheese, 1 cup of mozzarella cheese, 1/2 cup of Parmesan cheese, egg, and Italian seasoning. Mix until well combined.
In a medium saucepan over medium heat, melt the butter. Add minced garlic and sauté for 1 minute until fragrant.
Whisk in the heavy cream, salt, and pepper. Bring to a gentle simmer for 3-5 minutes until slightly thickened.
Remove the sauce from heat and whisk in the remaining 1 cup of Parmesan cheese until smooth.
Spread 1/2 cup of the Alfredo sauce onto the bottom of a 9x13 inch baking dish.
Stuff each cooked shell generously with the chicken mixture and place open-side up in the baking dish.
Pour the remaining Alfredo sauce evenly over the stuffed shells.
Sprinkle the remaining 1 cup of mozzarella cheese over the top.
Bake for 20 to 25 minutes, or until the cheese is melted and bubbling.
Garnish with chopped fresh parsley and serve immediately.