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.