Roughly chop the cranberries and place them in a medium bowl with the brown sugar. Let macerate, stirring occasionally with a wooden spoon, while you make the crust.
To make the crust, whisk together the dry ingredients in a large mixing bowl. Add the olive oil, water and vinegar and mix with a flexible spatula or wooden spoon. Knead the dough a few times in the bowl with your hands just to bring it all together.
Transfer the dough to a sheet of parchment paper and shape it into a flat disc in the center of the paper. Place another piece of parchment paper on top of it and begin rolling out the dough until it is between 1/8 and 1/4 of an inch thick and is in a round(ish) shape.
Place the rolled dough, still between the sheets of paper, on a baking sheet. Peel off the top sheet.
To make the filling, add the cornstarch and salt to the cranberries and stir until the cornstarch disappears. Add the vanilla and stir until combined.
Place the filling in the middle of the dough, leaving about a two-inch border. Gently fold up the edges, decoratively squeezing the dough folds together. And freeze the galette for about an hour.
Preheat the oven to 400°F.
To make the egg wash, whisk together the egg and salt in a small bowl.
Brush the edges of the galette with the egg wash, sprinkle with the Turbinado sugar and bake the pie for 40 to 45 minutes, rotating at the halfway point, until the filling is bubbly and the edges of the dough are nicely browned.
Slice the galette with a pizza cutter, if you have one, and serve with vanilla ice cream.