Preheat the oven to 350 degrees Fahrenheit and line an 8” x 8” square baking dish with parchment paper.

Add all of the ingredients for the crust and topping to a large mixing bowl and stir well until everything is combined and a thick dough forms. The dough should be somewhat crumbly and not over oily. Measure out ½ cup of the mixture and set it aside for later to reserve as the crumble topping.

Transfer the remaining crust mixture to the parchment-lined prepared 8-inch pan and press it into an even layer.

Use a fork to poke several holes into the crust, allowing the crust to bake evenly. Bake the crust for 15 to 20 minutes, or until it is just slightly golden brown.

While the crust is baking, prepare the blackberry filling mixture. To do so, transfer the blackberries, 2 tablespoons of water, and the granulated sugar (or pure maple syrup) to a saucepan. Cover the pan with a lid and heat it on the stove top over medium-high heat.

Allow the mixture to come to a full boil, stirring occasionally, and cook until the blackberries are very soft and much liquid is seeping out, about 8 to 10 minutes. Mash the blackberries using a spoon or fork until they reach your desired texture. Stir in the gluten-free all-purpose flour (or tapioca flour or cornstarch) and continue cooking without the lid until the blackberry filling is nice and thick, about another 2 to 3 minutes.

Once the crust is out of the oven, pour the blackberry filling mixture over it and spread it evenly over the crust to form the blackberry layer.

Sprinkle the reserved oatmeal crumble mixture on top. Bake for 25 to 32 minutes, or until the filling is thick and jammy and the topping is slightly golden-brown.

Remove the blackberry oatmeal bars from the oven and allow them to cool completely before slicing and serving.

If desired, make the optional glaze by stirring the powdered sugar and water (or lemon juice) together in a small bowl. Drizzle over the oat bars and enjoy!