1. Cut cauliflower into flowerets. Cut tough stalks from broccoli; trim tough sides; cut stalks into coins; cut head into flowerets.
2. Bring 6 cups water in large saucepan to boiling. Add cauliflower, broccoli and salt. Return to boiling; cook, stirring occasionally, until vegetables are tender-crisp, about 8 minutes. Drain.
3. Meanwhile, prepare the sauce. Melt the butter in a 2-quart saucepan over medium heat. Whisk in the flour and mustard until smooth; cook for 1 minute. Gradually stir in the milk until smooth. Bring to boiling. Remove the saucepan from the heat. Stir in cheese and hot-pepper sauce, stirring until cheese is melted. Spoon sauce over vegetables.