Preheat oven to 350°.
Heat a large nonstick skillet over medium-high heat. Sprinkle chicken with Cajun seasoning. Coat pan with cooking spray. Add chicken to pan, and cook 7 minutes on each side or until done. Cut chicken into 1/4-inch-thick slices; set aside.
Melt butter in a large Dutch oven over medium-high heat. Add bell pepper and next 4 ingredients (bell pepper through garlic); sauté 7 minutes or until tender. Sprinkle with flour, and cook 1 minute, stirring constantly. Stir in the Worcestershire and mustard; gradually add milk, stirring with a whisk. Bring to a boil; reduce heat, and simmer 3 minutes or until slightly thick. Remove from heat; stir in 3/4 cup green onions, 6 tablespoons cheese, and 3 tablespoons parsley. Add chicken and pasta to sauce mixture; toss well to combine.
Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray. Sprinkle with remaining 1/4 cup green onions, 2 tablespoons cheese, and 1 tablespoon parsley. Cover and bake at 350° for 20 minutes; uncover and bake an additional 5 minutes.