In a large saucepan, bring water to boiling. Add cauliflower and cook for 4 to 6 minutes or until crisp-tender. Drain cauliflower and set aside.
In another saucepan, cook mushrooms and green pepper in butter or margarine until pepper is tender. Stir in flour and salt. Add the milk all at once. Cook and stir until bubbly. Remove from heat. Stir in Swiss cheese and pimiento until cheese is melted.
In a 1-1/2-quart casserole, place half of the cauliflower. Cover with half of the sauce. Top with remaining cauliflower and sauce.
Bake, uncovered, in a 325 degree F oven for 15 minutes. Makes 8 servings.