Keeping macaroni warm, add sour cream. Meanwhile, preheat oven to 350. Prepare a 11"x7" baking dish with cooking spray; set aside. In a medium sized saucepan add milk, onions, flour, mustard, salt, red pepper flakes, and black pepper. Mix with a wire whisk until smooth. Cook over medium heat, stirring constantly until mixture boils and thickens. Remove from heat. Stir in cheese until melted. Add mixture to macaroni. Mix well. Spoon into prepared dish. In small bowl, combine bread crumbs and parmesan cheese. Mix well. Sprinkle evenly over top casserole. Bake for 25 minutes, or until edges are bubbly.