Heat oven to 350°F. Spray 2 1/2-quart casserole with cooking spray.
In 5-quart Dutch oven, heat 8 cups water to boiling. Add carrots; boil uncovered 10 minutes. Add cauliflower; boil 4 minutes longer. Drain; set aside.
Meanwhile, in 4-quart saucepan, melt 3 tablespoons of the butter. Cook onion in butter about 4 minutes, stirring occasionally, until softened. Stir in flour, salt and mustard with whisk; cook about 1 minute or until bubbly. Slowly stir in milk. Cook and stir until bubbly and slightly thickened. Remove from heat; stir in cheese until melted. Stir in carrots and cauliflower. Pour into casserole.
In small microwavable bowl, microwave remaining 3 tablespoons butter on High 1 minute or until melted; stir in bread crumbs. Sprinkle over vegetable mixture.
Bake uncovered 25 to 30 minutes or until bubbly around edges. Let stand 5 minutes before serving.