Wash cauliflower and break into flowerets. Cook, covered, in a small amount of boiling salted water 10 minutes or until crisptender; drain. Set aside.
Sauté onion and green pepper in butter in a large saucepan until tender; add flour, stirring well. Cook 1 minute, stirring constantly. Stir in cauliflower, tomatoes, salt, and pepper, mixing well. Cook over medium heat 3 minutes or until thoroughly heated.
Spoon mixture into a 1 1/2- quart casserole; top with cheese. Bake at 350° for 5 minutes or until cheese melts.