Preheat oven to 375°F (190°C).
Cook jumbo pasta shells until al dente, then drain and set aside.
In a skillet, melt butter and sauté garlic until fragrant. Add cooked chicken, Italian seasoning, salt, and pepper. Stir and let cool.
In a bowl, combine ricotta, mozzarella, Parmesan, and the chicken mixture to make the filling.
Prepare Alfredo sauce by melting butter in a saucepan. Add garlic and cook briefly. Stir in cream, nutmeg, salt, and pepper. Simmer for 5 minutes. Gradually add Parmesan until smooth.
Spread 1 cup of Alfredo sauce into a baking dish.
Stuff each shell with the chicken mixture and arrange in the dish.
Pour remaining Alfredo sauce over the shells.
Sprinkle mozzarella cheese on top.
Bake for 25 minutes until bubbly and golden.
This garlic butter chicken alfredo stuffed shells dish is best served hot and fresh.