Heat oven to 350°F. Line an 8-inch square pan with parchment paper.
Combine the shortbread base ingredients in a medium bowl, then transfer to your prepared baking pan. The easiest way to get the base evenly distributed in the pan is to add it in spoonfuls instead of all at once.
Use your fingers or the base of a drinking glass to press the batter firmly into the pan before baking. Remember to set aside ¼ of the base mixture (about 1 cup) for the crumble topping. Bake the crust for 8 - 10 minutes or until golden and slightly puffed.
Meanwhile, in a saucepan, combine diced apples with spices, salt, tapioca starch and brown sugar or maple syrup. Cook and stir over medium heat for 5 - 7 minutes or until thickened. Remove from heat.
Spread the apple filling evenly over the cooked base.
Sprinkle with pomegranate arils.
Top with the remaining crumble topping, pressing down gently to hold in place. Bake for 20 - 24 minutes at 350°F. or until light golden brown.
Let cool in pan for 15 minutes then carefully lift the parchment paper "handles" to transfer to a cooling rack. Slice into bars once completely cooled.
Remember to reserve one cup of the shortbread base for the crumble topping. It's easy to forget!It's best to store these crumble bars in the fridge. They will last for several days.These apple crumble bars freeze well, too. Store in an airtight container with a sheet of parchment paper between layers. They'll last for three months or so.