Break cauliflower into florets. Wash and then steam for 5 minutes. Drain. Put into a greased 2-quart (2 L) ovenproof casserole dish.
Sauté mushrooms (optional) and celery in butter. Blend in flour, mustard and salt. Stir well. Gradually stir in milk. Cook slowly for about 8 minutes or until thickened. Add cheese, stirring until melted. Pour sauce over the cauliflower.
Sprinkle cornflake crumbs on cauliflower. Bake at 350°F (180°C) for 30 minutes.