In a medium bowl, whisk together the rolled oats, brown sugar, flour, salt, and cinnamon. Use your fingertips to cut in the chilled cubed butter until the mixture starts to clump together. Cover and refrigerate while you prepare the crust and filling.

In a large bowl, whisk together the flour, salt, and cinnamon. In a stand mixer fitted with a paddle attachment, cream together the softened butter, granulated sugar, and brown sugar until creamy. Add the vanilla extract, and gradually mix in the dry ingredients on low speed. Once combined, press the dough evenly into a 9×13 inch oven-safe baking dish. Refrigerate for 20 minutes.

Preheat your oven to 375°F (190°C) and bake the chilled crust for 18 minutes. Once baked, remove it from the oven and allow it to cool briefly. Don't forget to reduce the oven temperature to 350°F (175°C).

While your crust cools, toss the sliced apples with sugar, cinnamon, and optional nutmeg in a large bowl. In a separate small bowl, whisk together the lemon juice and cornstarch before adding this mixture to the apples and tossing well. Transfer the prepared apple mixture to a large saucepan over medium heat. Cook for 5-8 minutes, stirring occasionally, until the apples become soft. Add water as necessary to prevent burning.

Spread the cooked apple filling evenly over the baked shortbread crust. Finally, sprinkle the chilled oat streusel topping evenly over the apples.

Bake the assembled bars at 350°F (175°C) for 30-35 minutes, or until the streusel is golden brown and the apples are bubbly. Allow the bars to cool completely before slicing them into 12 large bars. Serve plain or with a scoop of vanilla bean ice cream for extra indulgence.