In a large skillet over medium-high heat, melt the butter.Tip: It’s important to use a large skillet with a large surface area instead of a pot or saucepan, because the larger surface area speeds up the moisture evaporation. This ensures that a lot of the moisture released by the apples will have evaporated by the time the apples have softened, so that the final filling won't be too wet.

Add the diced apples, sugar, lemon juice, cinnamon and salt, and stir to combine.

Cook for 5-7 minutes over medium-high heat with occasional stirring, until the apples are softened and the juices have thickened into a thick, syrupy consistency.Tip: You want the apples softened but they should still have some texture to them. It’s better to err on the firm side with the apple pieces, as they will soften even further during baking.

Make sure that the apple pie filling has cooled completely to room temperature before you assemble the apple pie bars. Tip: You can prepare the apple pie filling a few days in advance and keep it in the fridge until needed.

Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line a 9-inch (23cm) square baking tin with parchment/baking paper.Tip: When lining the baking tin, I recommend leaving some parchment/baking paper overhang, as that will help with removing the bars from the tin later on.

In a large bowl, mix the softened butter, light brown sugar and vanilla together with a wooden spoon or rubber spatula until combined.Tip: Don’t cream or aerate the mixture – you don't want to incorporate air into the cookie dough, as that can make the bottom crust and the crumb topping puff up and/or spread too much, and it can result in apple pie bars that are too crumbly and delicate (note the absence of any raising agents for the same reason).

Sift in the gluten free flour blend, xanthan gum and salt.

Mix it all together until the dough starts clumping together, then give it a thorough knead to bing it together into a smooth ball with no patches of dry flour. The dough might crack in places, but it should hold together well without being too crumbly.

Press just over half of the shortbread cookie dough (about 380g) into the lined 9-inch (23cm) square baking tin. Smooth it out into an even layer, this is easiest done with a small offset spatula. The thickness of this bottom cookie dough layer should be about ¼-inch (7mm).

Sprinkle the ground walnuts evenly across the cookie dough crust, all the way to the edges.

Arrange the apple pie filling in an even layer on top of the ground walnuts, and drizzle over all the syrupy juices that the apples have released.

Crumble over the remaining shortbread cookie dough (about 315g). Try to get an even coverage, but it’s okay if a few apple pieces peak through here or there.

Bake at 350ºF (180ºC) for about 40-45 minutes or until the crumb topping is evenly golden brown.

Allow the bars to cool to room temperature before removing them from the baking tin (with the help of the overhanging parchment/baking paper) and slicing them into individual portions.Tip: It’s VERY important that the apple pie bars are cooled completely before you remove them from the baking tin. While they’re still warm or hot, they will be much crumblier and more delicate, so they will be more difficult to handle (they’re more likely to crack and crumble if you try to remove them while they’re still hot or warm).

The gluten free apple pie bars keep very well in a closed container at room temperature (or in a cool, dry place) for about 3-4 days.