Preheat broiler. Combine cornflakes and parmesan cheese for topping; set aside. Stir together flour, milk, broth, mustard and onion powder in microwavable bowl; mix well. Microwave cream sauce on High for 4-6 min. Stir well every 2 min. Cook until thick. Stir in cheddar cheese.
Spread broccoli into 9 x11 pan. Pour cheese sauce over broccoli. Add corn flakes and parmesan cheese topping.