Heat oven to 350°F. In large bowl, combine flour, brown sugar and margarine; beat at low speed until crumbly. Stir in nuts. Press 2 3/4 cups crumb mixture in bottom of ungreased 13x9-inch pan.
To remaining mixture, add cinnamon, baking soda, salt, sour cream, vanilla and egg; mix well. Stir in apples. Spoon evenly over crumb mixture in pan.
Bake at 350°F. for 30 to 40 minutes or until toothpick inserted in center comes out clean. Cool 30 minutes. Cut into squares. Serve warm or cool. If desired, serve with whipped cream or ice cream. Store in refrigerator.