• Grease a 9-inch square stoneware baking dish, preferably in a light color, lightly with butter or neutral oil. Set it aside.

Prepare the filling

• In a large mixing bowl, whisk together the sugar, tapioca starch, salt and ground cinnamon.
• Transfer about 3 tablespoons of this dry mixture to a small mixing bowl. Add the fresh raspberries to the dry ingredients and toss to coat them in it, then set the bowl aside.
• Add the fresh blackberries to the large mixing bowl with the remaining dry ingredients, and toss them gently to coat all the berries well in the dry mixture.
• Transfer the blackberry mixture to the prepared baking dish, and spread them gently into an even layer. Add any dry mixture that remains on the bottom of the bowl.
• Scatter the raspberries on top in an even layer, scattering any leftover dry ingredients on top. Set the baking dish aside.

Prepare the topping

• In a large mixing bowl, whisk together the flour blend, xanthan gum, baking powder, baking soda, salt and sugar.
• Add the slices of cold butter, and toss to coat in the dry ingredients. If any slices of butter weren’t thinly sliced, flatten them with floured fingers.
• Create a well in the center of the dry ingredients, and add the buttermilk to the center. For slightly drier biscuits that hold their shape better in the oven, reduce the buttermilk by up to 1/8 cup.
• Mix the buttermilk in to combine well. The mixture will be quite wet, unless you’ve reduced the buttermilk. Even then, it will come together pretty easily.
• If the topping seems like it isn’t very cold or the butter has melted at all, place the topping mixture in the refrigerator to chill.

Add the topping

• Use a large ice cream scoop or two large spoons to top the berry mixture in the baking dish with 6 relatively equal scoops of the pastry topping. Concentrate them mostly toward the center as the biscuits will spread in the oven toward the edges.

Bake

• Bake at 375°F for 20 minutes. Lower the heat to 350°F for another 20 minutes or until the filling is bubbling, the topping feels firm when pressed gently, and a toothpick inserted in the center of one of the biscuits (without entering the filling) comes out clean.
• Let cool for about 15 minutes in the pan before serving warm.