In large pot, melt 1/4 cup butter over medium heat. Stir in flour; cook 2 minutes on medium heat, stirring constantly. Whisk half and half into flour mixture; cook until thickened, stirring constantly. Reduce heat to low; stir in cheeses, green onion, 2 tablespoons parsley, salt, dry mustard and pepper. Stir in cooked pasta. Spoon pasta mixture into prepared pan.