Boil shells in salted water until al dente (about 10 minutes). Drain and cool on a baking sheet.
Heat oil in a skillet. Cook onion and bell pepper for 5 minutes. Add garlic, Cajun seasoning, paprika, salt, and pepper. Stir in chicken and heat through. Let cool slightly.
Mix the chicken mixture with ricotta, mozzarella, and Parmesan until combined.
Melt butter in a saucepan. Whisk in flour and cook 1 minute. Gradually add milk and cream. Stir in Cajun seasoning, garlic powder, salt, and pepper. Simmer 5–7 minutes until thick.
Preheat oven to 375°F (190°C). Spread half the sauce in a 9×13 dish. Stuff each shell with filling and place in dish. Pour remaining sauce on top. Add extra mozzarella if desired.
Cover with foil and bake for 20 minutes. Uncover and bake 10 more until golden and bubbling.
Let sit for 5 minutes. Garnish with parsley and serve.