Cook pasta according to package directions. Drain and set aside.
While pasta cooks, melt butter over medium heat in a Dutch oven. Add red bell pepper, celery, and onion. Cook until soft, about 5 to 7 minutes.
Add garlic and cook 2 more minutes.
Add crawfish (along with any liquid in the bag), Cajun seasoning, and black pepper and cook 2 to 3 minutes.
Sprinkle flour over crawfish mixture. Stir and cook 1 minute.
Stir in half-and-half. Cook until mixture comes to a simmer.
Add Velveeta, sour cream, and Parmesan cheese and cook over medium heat until cheese is melted.
Stir in cooked pasta, green onions, and parsley. Serve.
