Preheat your oven to 350°F (175°C). Grease and line an 8x8-inch square baking pan with parchment paper, leaving some overhang on two sides for easy removal.
In a bowl mix the cubed cold butter, brown sugar, salt, flour, oats, and cinnamon. Use either your hands or a pastry cutter. Once the mixture is combined well and has small clumps, set aside in the fridge for later.
In a medium bowl cream the flour, both sugars, vanilla extract, and salt together until soft cookie dough forms.
Spread the cookie dough on an 8x8-inch pan lined with parchment paper. Press to form an even layer. Bake in the oven for 15 minutes.
In a large bowl, toss the sliced apples with ground cinnamon, granulated sugar, nutmeg, and lemon juice. Spread the apple mixture evenly over the shortbread crust.
Sprinkle the prepared streusel topping over the apples, covering them evenly.
Place the baking pan in the preheated oven and bake for approximately 40-45 minutes or until the edges are golden brown and the apples are softened.
Remove the apple pie bars from the oven and let them cool completely in the pan on a wire rack. Top with powdered sugar, caramel, or enjoy as is!
