Preheat oven to 350°. Lightly butter a 13x9x2-inch baking pan.
Combine the flour, brown sugar, and butter in a large bowl. With an electric mixer on low speed, blend until the mixture is crumbly. Stir in the chopped walnuts. Press 2 2/3 cups of the crumb mixture in the prepared baking pan.
To the remaining crumb mixture, add the cinnamon, baking soda, salt, sour cream, vanilla, and egg. Mix until well blended. STir in the chopped apples. Spread the apple batter over the crumb layer.
Bake for 30 to 40 minutes, or until a toothpick or cake tester inserted in the center comes out clean. Cut into squares and serve warm with whipped topping or a scoop of vanilla ice cream.
Calories per serving: 368
Fat per serving: 18.4