Lightly spoon flour into measuring cup; level off. In large bowl, combine flour, brown sugar and butter; beat at low speed till crumbly. Stir in nuts. Press 2 3/4 cups of crumb mixture in bottom of ungreased 13x9" pan.
To remaining mixture, add cinnamon, baking soda, salt, sour cream, vanilla and egg; mix well. Stir in apples. Spoon evenly over base.
Bake at 350* for 30-40 minutes or till toothpick comes out clean.