Cook rice mix in a large skillet according to the package directions using 1 Tbsp margarine and 2 1/4 cups hot water. Remove mixture from skillet and set aside. Preheat oven to 350 degrees. Saute chicken and garlic powder for six minutes then add mushrooms, cook until done. Combine rice mixture, chicken mixture, sour cream, pepper, and soup in a bowl. Coat casserole dish with cooking spray then spoon mixture into dish. Combine cracker crumbs, melted margarine, and poppy sides. Stir well and sprinkle over mixture. Bake at 350 for 35 minutes.