• Preheat your oven to 325°F. Grease a 9-inch square baking pan, and line with crisscrossed sheets of unbleached parchment paper to make it easy to lift the bars out of the pan after baking.

Make the crumble topping.

• Combine all of the topping ingredients (8 tablespoons butter, 1/2 cup all purpose flour and xanthan gum, rolled oats, granulated and brown sugars, and salt), and blend with a fork until well-combined.
• Place in the refrigerator to chill until ready to use.

Make the crust.

• In a large bowl, place the all purpose flour, xanthan gum, oat flour, salt, baking powder and granulated sugar, and whisk to combine well. Add the oats, and mix to combine.
• Create a well in the center of the dry ingredients, and add the butter, egg and 1 tablespoon water, and mix to combine.
• The dough will be a bit dry, and you will have to knead it with your hands to get it to come together. If necessary for the dough to hold together, add a second tablespoon of water, 1 teaspoon at a time.
• Transfer the dough to the prepared baking pan, and press firmly into an even layer on the bottom of the pan.
• Cover with another sheet of unbleached parchment paper and smooth out the dough by pressing it down evenly.
• Place the pan in the center of the preheated oven and bake for 7 to 10 minutes or until it appears set. It will lose some of its shine.

Make the fruit layer and assemble.

• While the crust is parbaking, in a small bowl, place the blueberries, granulated sugar, oat flour, and salt, and toss to combine.
• After the crust is done parbaking, remove the pan from the oven and spread the blueberries out on top of the crust in a single, even layer.
• Remove the crumble topping from the refrigerator and fluff it with a fork to break it up into irregular pieces.
• Sprinkle the crumble topping in an even layer on top of the blueberries, and press gently to help it adhere to the blueberries in the filling.
• Return the pan to the oven, and bake until the crust is beginning to brown around the edges, a toothpick inserted into it comes out clean and the crumb topping is lightly golden brown (about 15 minutes).
• Remove the pan from the oven and allow to cool completely in the baking pan before lifting the bars out of the pan with the parchment paper.
• Slice into 12 equal pieces. Store each bar for later use wrapped separately, in the refrigerator or freezer.