In a medium saucepan, saute green pepper and mushrooms in butter for 2 minutes. Add flour; gradually stir in milk. Bring to a boil; boil for 2 minutes, stirring constantly. Remove from the heat; stir in cheese until melted. Add pimientos and salt.
Place half of the cauliflower in a greased 2 quart baking dish; top with half of the sauce. Repeat layers.
Bake, uncovered, at 325F for 25 minutes or until bubbly. Sprinkle with paprika if desired.
8 to 10 servings.