Preheat oven to 450. Grease a 2 quart casserole. Mix together yogurt or sour cream, 1/2 cup sugar, salt, vanilla, and 1/3 cup flour. Pour into casserole. Drop apples into mixture. Bake 10 minutes at 450 and another 40 minutes at 350. Stir together brown sugar, sugar, cinnamon, flour, butter and walnuts and top baked pie. Bake again another 15 minutes at 350.