Preheat oven to 375°F and line an 8x8 baking dish with parchment paper. Leave paper hanging over the sides to make bars easier to remove from baking dish.

In a bowl of a stand mixer, add gluten free flour, sugar, and salt and whisk until combined.

Using the paddle attachment, mix butter one chunk at a time on low speed until the mixture becomes crumbly and pulls away from the sides of the bowl. This will only take about 3 minutes.

Reserve ⅓ cup of the dough. Set aside.

Pour the remaining dough into the prepared baking dish and press down firmly using fingers or back of a spatula. Bake in preheated oven for 15 minutes.

As the shortbread base is baking, sprinkle cinnamon over chopped apples and mix until evenly coated.

In a separate bowl, add reserved shortbread base, brown sugar, and oats and stir together until evenly mixed. Use your fingers to break apart any lumps of brown sugar.

After baking the shortbread base, pour apples on top, then sprinkle the oat crumble over the apples.

Bake for 22 minutes then cover the baking dish and continue baking for 10 more minutes. Turn the oven off and leave baking dish in the oven to continue baking for 5 more minutes before removing.

Cool to room temperature before removing bars from dish and cutting.

Enjoy!