Preheat oven to 350°. In a 6-qt. stockpot, cook macaroni according to package directions, adding broccoli, cauliflower, carrots and celery during the last 6 minutes of cooking. Drain; transfer to a greased 13x9-in. baking dish., Meanwhile, in a large saucepan, heat butter over medium-high heat; saute onion until tender. Stir in flour until blended. Gradually stir in milk and broth; bring to a boil. Cook and stir until thickened, about 2 minutes; stir in cheese, mustard, salt and pepper. , Add to macaroni mixture, stirring to coat; sprinkle with paprika. Bake, uncovered, until heated through, 15-20 minutes.