Preheat the oven. Preheat your oven to 300°F. Line an 8 x 8-inch baking dish with parchment paper, leaving an overhang on at least two sides for easy removal.
Make the shortbread crust. In a medium bowl, whisk together the melted butter, monk fruit sweetener, vanilla extract, and salt. Add the almond flour and mix until a soft dough forms. Press the crust mixture firmly and evenly into the bottom of the prepared baking dish.
Prebake the crust. Bake for 15 minutes while you prepare the apple filling and streusel topping.
Prepare the apple filling. Peel, core, and thinly slice the Granny Smith apples using a sharp knife or mandoline. In a large bowl, combine the sliced apples, almond flour, monk fruit sweetener, cinnamon, and nutmeg. Gently toss until the apples are evenly coated.
Make the streusel topping. In a separate medium bowl, combine the gluten-free oats, coconut sugar, cinnamon, and almond flour. Add the softened coconut oil and use a fork or pastry cutter to work it into the dry ingredients until coarse crumbs form. Refrigerate for 10 minutes to help the crumble topping firm up.
Assemble the apple pie bars. Remove the prebaked crust from the oven and increase the temperature to 350°F. Arrange the apple filling in an even layer over the warm crust, gently pressing the apple slices down to create a compact filling.
Add the streusel. Remove the streusel topping from the refrigerator and sprinkle it evenly over the apple mixture, breaking apart any large clumps with your fingers.
Bake. Return the baking dish to the oven and bake for 30–35 minutes, or until the streusel topping is golden brown and the apple filling is tender.
Cool and serve. Allow the apple pie bars to cool in the pan on a wire rack for at least 20 minutes. For cleaner slices, refrigerate until thoroughly chilled before cutting into squares. Drizzle with homemade salted caramel sauce before serving.