Make the Pastry: In a large bowl, whisk together flour, sugar, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Gradually add ice water, 1 tablespoon at a time, mixing just until the dough comes together. Form into a disc, wrap in plastic wrap, and chill for at least 30 minutes.

Prepare the Filling: In a medium bowl, combine Greek yogurt, honey, lemon zest, and vanilla extract. Stir until smooth. Gently fold in the blueberries. If using cornstarch, whisk it with the lemon juice in a small bowl to create a slurry, then gently stir into the yogurt-berry mixture.

Assemble the Galette: Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper. On a lightly floured surface, roll out the chilled dough into a rough 12-inch circle. Carefully transfer the dough to the prepared baking sheet.

Spoon the Greek yogurt and blueberry filling onto the center of the dough, leaving a 2-inch border. Gently fold the edges of the dough up and over the filling, pleating as you go to create a rustic crust. Brush the folded crust edges with the beaten egg wash and sprinkle with coarse sugar, if desired.

Bake: Bake for 30-40 minutes, or until the crust is golden brown and the filling is slightly set and bubbly. If the crust starts browning too quickly, you can loosely tent the galette with foil.

Cool: Let the galette cool on the baking sheet for at least 15 minutes before slicing and serving.