Preheat oven to 350°F. Line a 9×13 pan with parchment paper, leaving extra hanging over the ends, spray paper and sides of dish with cooking spray containing flour.

In a large mixing bowl, beat together the butter and brown sugar until light and fluffy, about 2-3 minutes. Add vanilla, mix until combined. Add flour and mix until combined and dough is crumbly. Press dough evenly into bottom of prepared pan.

Bake for 12-15 minutes, until dough is slightly puffed. Remove from oven and cool on a wire rack while preparing apples and topping. Maintain oven temperature.

Evenly layer sliced apples over pre-baked crust. Combine brown sugar, cinnamon, allspice and nutmeg. Sprinkle evenly over apples.

In a medium bowl, combine oats, flour, walnuts, cinnamon, sugar, and salt. Add butter and cut with a pastry cutter or form until mixture forms pea-sized crumbs.

Sprinkle streusel evenly over apple filling.

Bake bars at 350°F for 25 minutes, or until streusel is golden and apples are softened.

Cool bars completely on a wire cooling rack. Cut into squares and serve.