Start by making the crumble crust and topping. Add the melted coconut oil, flax egg, brown sugar, cane sugar, and almond extract to a bowl and mix together. Then add in the oats and mix to coat in the wet ingredients. Finally, add in the gluten-free flour, baking powder, and salt. Stir together until a thick somewhat crumbly dough forms.
Line an 8*8 pan with parchment paper and press ¾ of the dough into the bottom of the pan. Then, add the pan to the oven and bake for 15 minutes. Add the ¼ cup of slivered almonds to the remaining ¼ of dough for the topping and set aside.
While the crust is baking, make the blueberry filling. First add the arrowroot powder, water, and lemon juice to a small sauce pan and mix together to dissolve the arrowroot powder into a slurry. Then, add in the blueberries and sugar and mix together. Turn on the heat and bring to a simmer. Once simmering, the filling should thicken quite a bit- this takes about 5 minutes and it is best to stir frequently while it is cooking. Once you see it thickening, immediately remove the pan from the heat and set aside.
Remove the pan from the oven and top with the blueberry filling. Use a spatula to evenly spread the filling out over the crust.
Next, add the remaining ¼ of dough with the almonds on top of the blueberry filling. Then, add the pan back to the oven and bake for another 15-20 minutes until the crust is cooked and just beginning to brown.
Remove from the oven and let cool completely before slicing. To make slicing easier, lift the parchment paper up to easily remove the crumble bars. Then, slice into 9 even pieces and serve!