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-quart casserole. In a small bowl combine topping ingredients (1/2 cup margarine, the crushed crackers and 2 teaspoons poppy seed). Sprinkle over the chicken. Bake at 350 degrees for 20 minutes.