Place the flour, 1 1/2 tablespoons sugar and salt in the bowl of a food processor.
Process until combined.
Add the cold butter cubes to the dry ingredients.
Pulse 20-30 times until the mixture resembles a coarse meal and the butter is in pea-sized chunks.
Combine the milk and vinegar in a measuring cup.
With the processor running, add the milk through the feed tube until combined and the mixture begins to clump and form a ball. This will not take long! Be careful not to over mix -- as soon as you can pinch the dough together and it holds, it's done.
Pour the dough out onto a board.
Form into a disc and wrap in plastic wrap, then chill for a minimum of 30 minutes.
Once chilled, preheat the oven to 375 degrees. Place the dough on a lightly floured surface and roll until 12" wide (it will be about 1/8" thick).
Line a large baking sheet with parchment paper. Roll the dough loosely around a rolling pin, then unroll over the prepared baking sheet and set aside.
Place the blueberries, sugar, cornstarch, lemon zest and juice in medium bowl.
Stir the blueberry mixture until well combined.
Gently pour the blueberry filling in the center of the galette dough, leaving a 1 1/2 - 2" border.
Fold the edges of the dough over the filling, overlapping it in sections.
Brush the dough evenly with the egg wash.
Bake the galette for 45-50 minutes, until the crust is golden brown and the filling is bubbly. Cover with foil if the dough begins to brown too quickly. To check the dough for doneness, use a spatula to lift part of the galette. The dough should be golden brown underneath as well.
Cool on a wire rack for about 10 minutes, and enjoy!