Combine apple slices and 1/2 cup sugar in bottom of a 2-quart casserole. Mix butter, oats, flour, 1/2 cup sugar and nuts. Sprinkle butter and oat mixture over apples. Bake in preheated 350 degree oven for about 1 hour until topping is browned and apples are tender. Serve warm. This is delicious topped with a scoop of vanilla ice cream.