Bring a large pot of salted water to a gentle boil and add the jumbo shells. Cook until just al dente, drain, and transfer to a baking sheet to cool slightly and prevent sticking.

Meanwhile, heat the olive oil in a skillet over medium heat and sauté the chopped onion and red bell pepper until softened, about five minutes. Add the minced garlic and cook one more minute until fragrant. Stir in the shredded chicken, Cajun seasoning, paprika, salt and black pepper, then remove from the heat. Transfer the mixture to a mixing bowl and fold in the ricotta, mozzarella and Parmesan until evenly combined.

In a separate saucepan, melt the butter over medium heat, whisk in the flour and cook for one minute. Gradually pour in the milk and heavy cream, whisking continuously to keep the sauce smooth. Stir in the Cajun seasoning and garlic powder, season with salt and pepper, and let the sauce simmer until it thickens enough to coat the back of a spoon, about five to seven minutes.

Preheat the oven to 375°F (190°C). Spread half of the cream sauce in the bottom of a 9×13-inch baking dish. Stuff each cooled pasta shell with the chicken-cheese filling and arrange them in the dish. Pour the remaining sauce evenly over the shells and sprinkle additional mozzarella on top. Cover with foil and bake for twenty minutes, then remove the foil and bake another ten minutes until the cheese is bubbly and lightly golden.

Allow the dish to rest for five minutes before serving, then garnish with chopped parsley or green onions if desired.