In a large bowl, whisk the flour blend, xanthan gum, baking powder, and salt to combine well.
Add the butter, and toss to coat it in the dry ingredients. Flatten each chunk of butter between your thumb and forefinger until it’s about ⅛ inch thick.
Create a well in the center of the dry ingredients and slowly add the water, taking care to filter out the ice cubes and mixing gently until the dough begins to come together.
If there are any portions of the dough that are especially dry, try to isolate them in the mixing bowl, drizzle in more water by the teaspoonful on only those parts, and then mix to moisten them.
Turn the dough out onto a sheet of plastic wrap and press it into a disc as you close the plastic wrap around the dough. It will seem rough and uneven.
Place the dough in the refrigerator to chill for 30 minutes or until firm while you make the filling.
Prepare the filling
Slice each hulled strawberry thickly in cross-section from top to bottom. Most of the slices should be able 1/4-inch thick, but some can be smaller.
Place the strawberry slices in a large bowl, add the vinegar and toss gently to combine.
Add the tapioca starch, granulated sugar, and the salt, and mix gently to combine.
Set the bowl aside to allow the juices to be drawn out of the strawberries while you roll out the pie crust.
Shape and fill the galette
Remove the pie crust from the refrigerator, and unwrap it. Sprinkle a clean surface with gluten free flour, place the crust on top, and sprinkle the top with more flour.
Use a rolling pin to press from the center out and roll the dough into a round about 12-inches in diameter. It will be about 1/4-inch thick, and will have irregular edges.
Transfer the crust to a large sheet of parchment paper and trim any especially rough edges.
Use a pastry brush or your fingers to brush the entire surface of the rolled out crust evenly with a thin layer of honey or syrup.
Leaving a clean 2-inch border all around, place half of the strawberry mixture in the center of the dough, piling the berries a bit toward the center.
Fold the dough toward the center, and over about one-quarter of the strawberry mixture in overlapping pleats where necessary.
Brush the sides and exposed strawberries with more honey or the egg wash, and press gently to help the crust adhere to itself and the filling.
Sprinkle the top of the galette all over with the optional coarse sugar. Slide a small cutting board under the parchment paper with the galette, and place it in the refrigerator to chill while you preheat the oven.
Preheat your oven to 375°F.
Bake
Transfer the raw galette with the parchment paper liner to a quarter sheet or jelly roll pan.
Place the baking sheet in the center of the preheated oven.
Bake until the strawberries are softened and the exposed pie crust is lightly golden brown all over, about 35 minutes.
Remove the baking sheet from the oven and use gentle pressure to settle down any of the pastry that has risen up above the edges.
Cool and serve
Allow the galette to cool on the baking sheet for a few minutes or until stable enough to transfer to a wire reack. Allow to cool until the filling is mostly set, about 20 minutes. Use a pizza cutter, pastry wheel or sharp knife to slice into 8 equal wedges and serve warm.
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Notes
Flour blendsMy favorite gluten free flour blends are Better Batter's original blend gluten free flour and Nicole's Best multipurpose blend. Bob's Red Mill 1-to-1 Gluten Free Baking Flour should also work, but you’ll need to add an extra ¾ teaspoon xanthan gum.To make your own blend using one of my “mock” recipes, please see the all purpose gluten free flour blends page.