Prepare the Crust: In a food processor, pulse the flour, sugar, and salt. Add the cold butter cubes and pulse until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining. Gradually drizzle in the ice water while pulsing until the dough just begins to come together.

Shape and Chill Dough: Turn the dough out onto a lightly floured surface, gather it into a disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour.

Prepare the Filling: In a large bowl, gently toss together the chopped rhubarb, blueberries, sugar, cornstarch, lemon zest, and vanilla extract until the fruit is evenly coated. Set aside while you roll the dough.

Assemble the Galette: Preheat oven to 400°F (200°C). On a sheet of parchment paper, roll the chilled dough into a rough 14-inch circle (it doesn't need to be perfect). Transfer the parchment and dough onto a baking sheet.

Fill and Fold: Mound the fruit mixture in the center of the dough, leaving a 2-inch border clear around the edge. Fold the border up and over the edges of the fruit, pleating the dough as necessary to form a rustic border. Brush the folded crust edges with the egg wash and sprinkle generously with turbinado sugar.

Bake: Bake for 40 to 45 minutes, or until the crust is deep golden brown and the filling is bubbling thickly. If the crust begins to brown too quickly, loosely tent it with aluminum foil.

Cool and Serve: Allow the galette to cool on the baking sheet for at least 20 minutes before slicing and serving. This allows the filling to set properly.