In a medium bowl, mix oat flour, white rice flour, brown rice flour, quinoa flour, tapioca flour, xanthum gum, and salt together.

Using a pastry cutter, incorporate the butter into the flour. When the pastry cutter gets clumped up, just use your fingers to break up the dough. Continue cutting the butter until the butter pieces are no bigger than the size of peas. You can also do this step by using a food processor or your hands to mix ingredients until the butter pieces are the size of peas.

Add 3 or 4 tablespoons of cold water to the bowl and stir ingredients with a fork or with your hands. You should see a very crumbly dough. Squeeze a small handful of dough together with your hands. If the dough stays together in one piece, your job is done. If the dough is still crumbly, add another tablespoon of water and stir. Keep doing this until your dough can clump up together.

Gather all the dough pieces together in a ball and flatten it into a disc. Wrap the dough in plastic wrap and refrigerate for 30 minutes.

While the dough is chilling, slice the tomatoes into 1/4-inch thick slices. Lay them over a dish or a baking sheet that is lined with paper towels. Sprinkle a pinch of kosher salt on top and set aside.

Prepare the kale pesto by blitzing all the ingredients together until you get a smooth mixture.

Preheat oven to 400 degrees F (205 degrees C).

Lay a sheet of parchment paper on a work surface. Sprinkle some white rice flour on top and flour your rolling pin. Take the dough out of the refrigerator and roll it until you get a circle that is about 12 inches wide. If the dough starts sticking to the pin, flour the pin more or sprinkle some flour directly on the dough. Trim off the uneven edges of the dough if you like. Slide the parchment paper on top of the baking sheet. I find it easiest to transfer the dough to the baking sheet at this stage.

Spread the pesto all over the dough, making sure to leave about 1 1/2 inches of space around the perimeter. Pat the tomato slices dry and lay them on top of the pesto spread. Sprinkle on corn kernels and parmesan cheese.

Fold in the edges of the galette. The dough was a bit crumbly for me and there were a few cracks here and there. I pinched the dough together along these cracks to close it up.

Crack the egg and beat it. Brush egg wash on the galette crust.

Bake for 35 to 40 minutes, until the crust is golden brown and the cheesy is bubbly.

Let galette cool for 15 minutes before serving.