Preheat oven to 350°F. Grease 9 x 13" baking dish.
In a large bowl combine all crust ingredients. Mix until crumbly. Press half of crumb mixture in greased pan. Bake for 8 minutes.
In a small saucepan over medium heat, combine caramel topping an flour. Stir constantly and bring to a boil. Boil 3 to 5 minutes or until mixture thickens slightly, stirring constantly.
Sprinkle apples (and nuts if using them) onto the warm base. Pour caramel mixture evenly over top. Sprinkle reserved crumbs evenly over the top. Bake 20 to 25 minutes or until golden brown. Cool completely before cutting. Refrigerate for easier cutting.