Preheat your oven to 375°F. Bring a large pot of salted water to a boil and cook the jumbo shells until just al dente. Drain the shells and let them cool slightly so they are easier to handle.
Heat the olive oil in a large skillet over medium heat. Add the cooked chicken, Cajun seasoning, garlic powder, onion powder, black pepper, and smoked paprika. Stir until the chicken is evenly coated and warmed through, about 2 to 3 minutes.
In a large mixing bowl, combine the seasoned chicken, ricotta cheese, 1 cup mozzarella cheese, 1/2 cup Parmesan cheese, egg, 1/4 cup Alfredo sauce, and parsley if using. Mix until the filling is creamy and evenly combined.
Spread about 1 cup of Alfredo sauce across the bottom of a 9x13 baking dish. Fill each cooked shell with the Cajun chicken Alfredo filling and place the shells in the dish, open side up.
Pour the remaining Alfredo sauce over the stuffed shells. Sprinkle extra mozzarella, Parmesan, and a light pinch of Cajun seasoning over the top.
Cover the baking dish with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes, or until the cheese is melted and the sauce is bubbling around the edges.
Let the stuffed shells rest for 5 minutes before serving. Garnish with chopped parsley, extra Parmesan, or crushed red pepper if desired.