In a large skillet, melt the butter over medium heat. Add the onions, bell peppers, celery, and garlic. Cook until tender, about 5 minutes. Add the flour and Cajun seasoning and cook for 2 minutes, stirring frequently. Stir in the milk and cook until slightly thickened, stirring constantly, 5 to 6 minutes. Add the soup and cheese, stirring until the cheese is melted. Add the crawfish, and cook until warmed through. Serve over the fettuccini. Garnish with chives, if desired.