Preheat oven to 375. Coat a 9 x 13 inch baking dish with nonstick spray.
For sauce, whisk together milk, sour cream, Dijon, Worcestershire, salt, black pepper and cayenne pepper.
For mac 'n cheese, mix cheddar and American cheese; reserve 2 cups. In prepared dish, layer half the macaroni, then top with half the carrot, celery, scallions and cheese mixture. Repeat layering. Pour sauce over casserole, cover with foil and bake, stirring every 15 minutes, til bubbly and pasta is al dente, about 45 minutes.
Remove foil and sprinkle Parmesan and reserved cheddar and American cheese mixture over top of casserole. Return casserole to oven and bake til cheese melts, about 10 minutes. Remove from oven and let sit 5 minutes. Garnish casserole with scallion greens if desired. Serves 8