Cook the fettuccine according to package directions in a large pot of salted boiling water until al dente; drain and set aside.
While the pasta cooks, melt the butter in a Dutch oven or large skillet over medium heat.
Add the chopped red bell pepper, celery, and yellow onion; cook, stirring occasionally, until the vegetables are softened, about 5 to 7 minutes.
Add the minced garlic and cook for about 2 minutes until fragrant.
Stir in the defrosted crawfish (including any liquid from the bag), the Cajun seasoning, and the black pepper; cook 2 to 3 minutes to heat through.
Sprinkle the flour over the crawfish mixture and stir constantly for 1 minute to cook the flour.
Slowly pour in the half-and-half while stirring and bring the mixture to a simmer.
Add the cubed Velveeta, sour cream, and grated Parmesan; cook over medium heat, stirring, until all the cheese is melted and the sauce is smooth.
Toss the cooked fettuccine into the sauce until evenly coated, then stir in the sliced green onion greens and chopped parsley; serve immediately.