Preheat the oven to 375°F.
Cook the jumbo shells in salted water until al dente. Drain and lay them out so they do not stick together.
In a large bowl, mix shredded chicken, ricotta, 1 cup mozzarella, ¼ cup parmesan, garlic, Italian seasoning, salt, pepper, and ½ cup Alfredo sauce.
Spread 1 cup Alfredo sauce in the bottom of a 9x13-inch baking dish.
Spoon the chicken filling into each shell and place the shells open-side up in the baking dish.
Spoon the remaining Alfredo sauce over the shells.
Sprinkle with the remaining ½ cup mozzarella and ¼ cup parmesan.
Cover with foil and bake for 20 minutes.
Remove foil and bake for another 8–10 minutes, until the cheese is melted and the sauce is bubbling.
Rest for 5 minutes, garnish with parsley if desired, and serve warm.