Boil macaroni. Preheat oven to 350°F. In small bowl, combine flour, salt, mustard, pepper, and cayenne. Melt butter in skillet over medium heat. Add scallions and saute about 2 minutes, until softened. Add flour mixture and cook for 3 minutes. Gradually whisk in milk. Cook 15 minutes, stirrings constantly until thickened. Remove from heat.
Stir in 1 1/2 cup cheese, and sour cream (if using). Add macaroni. Pour into greased 1 1/2 qt. baking dish. Top with remaining cheese and sprinkle with bread crumbs.
Bake 25-30 minutes, or until golden brown. Let sit 5 minutes before serving.