Preheat oven to 325°F. Lightly grease and flour an 8-inch square baking pan. Set aside.

Shortbread crust:

1. In a medium bowl, sift or whisk together flour and salt.
2. In a large mixing bowl, cream butter and sugar with a hand mixer until well combined.
3. Add flour mixture and stir until a stiff dough forms.
4. Spread evenly in the prepared pan. Bake for 5–6 minutes.

Apple filling:

1. In a medium bowl, mix apples, flour, brown sugar, cinnamon and nutmeg.
2. Remove pan from oven and increase oven temperature to 350°F.
3. Spread apple mixture evenly over crust. Set aside.

Streusel topping:

1. In a medium bowl, mix flour, oats, brown sugar and cinnamon.
2. Cut in cold butter using a pastry cutter, two knives or your fingers until coarse crumbs form, about the size of small peas.
3. Fold in walnuts. Sprinkle topping evenly over apples.
4. Return pan to oven and bake 35 minutes, or until lightly browned and bubbly.

Tip: If the top browns too quickly, cover with foil and continue baking until apples are tender.

Optional: Serve warm with whipped cream or vanilla ice cream.