Cook macaroni according to package directions, adding the broccoli, cauliflowerets, carrots and celery during the last 6 minutes. Drain; transfer to a greased 13-in. x 9-in. baking dish.
Meanwhile, in a Dutch oven, saute onion in butter until tender. Sprinkle with flour; stir until blended. Gradually stir in milk and broth. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in the cheese, mustard, salt and pepper.
Pour over macaroni mixture; stir to coat. Sprinkle with paprika. Bake, uncovered, at 350° for 15-20 minutes or until heated through. Yield: 12 servings.