Cheesy Broccoli

1 large broccoli head
2 Tbs butter
2 Tbs unbleached white flour
1/4 tsp dry mustard powder
1/4 tsp cayenne pepper
salt, pepper to taste 
1 cup skim milk
1 1/2 cups sharp cheddar cheese, grated

Mix flour, mustard powder, cayenne, salt, pepper. Steam broccoli. Melt butter in a medium saucepan. Whisk in flour mixture. Gradually stir in milk, whisking over med-low heat until thickened. Add cheese, stirring til melted.