Preheat the oven to 350°. Spray the bottom of an 8" square baking pan with vegetable spray and line with parchment paper, cut to the same dimensions as the pan. Leave extra paper on either side to act as a sling to remove the bars later.
Finely chop the walnuts in a mini food processor, pulsing several times until they resemble very fine crumbs.
In a medium bowl, combine the whole wheat flour, powdered sugar, walnuts, cinnamon and salt. Whisk together and add the cubes of butter. Work the mixture with your fingers until you have a rough crumbly dough that holds together when you pinch it between your fingers.
Transfer the whole wheat pastry to the baking pan and press it evenly across the bottom of the pan. Prick it several times with a fork and bake for 20 minutes or until the crust is light brown and set. Set aside.
FOR THE APPLE FILLING:
In a medium bowl, combine the cornstarch, brown sugar, cinnamon and nutmeg. Whisk until well combined.
Peel and quarter the apples. Cut out core and seeds then slice the apples very thinly from tip to tip. Add the apples to the brown sugar mixture and toss with clean hands until very well coated. Set aside.
FOR THE WALNUT STREUSEL:
Add the oats, brown sugar, cinnamon, whole wheat flour, salt, butter and walnuts to a small bowl. Work it with your fingers, pressing the butter with the other ingredients until you have a crumbly, but cohesive streusel mixture.
ASSEMBLE THE APPLE BARS
Spread the apples over the par-baked pastry, evenly and to the edge. (Do not add any liquids collected in the bottom of the bowl). Sprinkle the streusel evenly over the apples.
Bake until topping is golden brown and crisp, about 30-35 minutes.
Remove from the oven and cool to room temperature.
FOR GLAZE:
In a small bowl combine the powdered sugar, salt, vanilla and milk or cream and mix well until smooth and creamy. To make it thicker, add another spoonful or two of sugar. For thinner consistency, add an additional teaspoon or two of milk.
Run a sharp knife around the edges of the apple bars. Use the sling to transfer the apple squares to a cutting board. Drizzle the glaze over the cooled apple bars and allow them to rest for about 20-30 minutes so the glaze can "set".
Cut the bars into 16 squares and serve. Apple crumble bars can be stored in the refrigerator in an airtight container for 3-5 days.