If fresh cauliflower is used, remove the leaves. Soak in cold salted water 15 minutes. Drain. Separate into flowerets; cook in boiling salted water about 10 minutes, or until tender but firm. If frozen cauliflower is used, follow the instructions on the package. Drain. While the cauliflower is cooking, melt the butter in a skillet. Saute the mushrooms 5 minutes. Sprinkle with the flour, salt and pepper. Stir in the milk. Cook over low heat, stirring constantly. Pour over the cauliflower and serve. Serves 6; 35 calories per serving.