Cook whole head of cauliflower. (Heat salted water. Add cauliflower, cover and heat to boiling. Cook 20 - 25 minutes.)
Place cauliflower in casserole dish.
In saucepan, heat butter until melted. Blend in flour, mustard, salt and pepper. Cook over low heat, stirring constantly until mixture is smooth and bubbly. Remove from heat, stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.
Stir in cheese and pepper sauce. Stir over low heat until cheese is melted.