In large bowl combine flour, brown sugar and butter. Blend at low speed until crumbly. Stir in nuts. Press 2-2/3 C. of crumb mixture into ungreased 9x13-in. pan. To remaining mixture add cinnamon, soda, salt, sour cream, vanilla and egg. Blend well. Stir in apples (peeled and finely chopped); spoon evenly over base. Bake 25-35 min. in 350 degree oven or until toothpick comes out clean. Cut into squares and serve with whipped cream.