In a skillet, melt margarine. Saute mushrooms until tender. Stir in chicken, soup, sour cream and pimiento; mix well. Spoon mixture into a greased 2 litre casserole. In a small bowl combine topping ingredients ( half cup margarine, the crushed crackers and 2 teaspoons poppy seed). Sprinkle over the chicken. Bake at 180ºC or 20 minutes.