From: Art of Gluten-Free Baking (http://artofglutenfreebaking.com)
3 cups (420 g) Jeanne’s Gluten-Free All-Purpose Flour mix
2 teaspoons baking powder
½ teaspoon salt
¾ cup (150 g) granulated sugar
Zest and juice of ½ to 1 lemon (I used half, but you can use more)
1 cup (2 sticks; 225 g) unsalted butter, cold and but into pieces
1 extra-large egg
4 cups (590 g) fresh, whole blueberries
¼ cup (60 ml) honey
4 teaspoons tapioca flour (or cornstarch or potato starch)
Preheat oven to 375 degrees F/190 degrees C/Gas Mark 5. Grease a 9 x 13 inch/23 cm x 33 cm pan with butter or oil.
In a medium bowl, gently mixed together the blueberries and lemon juice. Add the honey and gently stir until the berries are covered. Add the tapioca flour one teaspoon at a time, and gently stir until the berries are covered.
If using a food processor
In the bowl of a food processor, place flour, baking powder, salt, sugar, and lemon zest. Pulse until combined. Add butter pieces and pulse until the mixture looks like wet sand combined with varying-sized pebbles. Add the egg and pulse until the mixture comes together a bit (it will still be crumbly).
If doing by hand
In a large bowl place flour, salt, sugar, and lemon zest and mix until combined. Add butter pieces and using your fingers or a pastry cutter, cut butter into the flour mixture until is looks like a combination of wet sand mixed with pebbles. Add the egg and mix until the mixture comes together a bit (it will still be crumbly).
Pat half the dough into the prepared pan. Sprinkle the blueberry mixture over the dough. Crumble the remaining dough over the blueberries to cover as evenly as possible.
Bake at 375 degrees F for about 55 minutes (until top starts to brown). Remove from oven and let cool completely before cutting. Store lightly covered at room temperature for up to 5 days.