Step 1In a large bowl, whisk together the flour, salt and sugar. Cut the fat into tablespoon-sized pieces and add them to the flour mixture. Toss the fat lumps around until they are coated with flour, then use a pastry cutter or your fingers to combine the fat with the flour until the fat is reduced to uneven crumbles, some as large as an almond and others as small as peas. Drizzle over the water and vinegar, and gently stir until the mass comes together (if using a high-fat butter, you might need up to 5 ounces of water). Don't worry if the dough is a little shaggy; it's all right as long as it sticks together.
Step 2Dump the mixture out onto your work surface. Use a bench scraper to gather the crumbs into the mass of dough. Use the heel of your hand to smear the dough away from you a third at a time. You are creating flat layers of flour and butter. After the dough is smeared out gather it back together with the bench scraper, using the scraper to layer the smears on top of each other, creating a mass of dough. Do it again. The dough should come together nicely, but you should still see pieces of butter.
Step 3Divide the dough in half and form into flat disks. Wrap each in plastic wrap and refrigerate until well-chilled, at least an hour and up to two days. You can also freeze the dough for up to 1 month.
Step 1Heat the oven to 425 degrees.
Step 2On a lightly floured surface, roll out one of the disks of dough until it is about 2 inches larger in diameter than the baking plate. Fit the dough into the plate and refrigerate. (If the dough is very firm, whack it a few times with the rolling pin to soften so the dough is easier to roll.) Roll out the second disk to make a top crust that is slightly larger than the diameter than the top of the pie plate. Cut out star and moon shapes from the top crust, saving the cutouts for decoration. (If you want to decorate the top with just cutouts, instead of a lattice, cut out more shapes as desired.) Refrigerate the top crust and cutouts.
Step 3In a large bowl, toss together blueberries, sugar, salt, lemon juice and cinnamon. Taste the mixture. If you think the berries need more sugar. add 1 tablespoon at a time, tasting after each addition until they taste good to you. Add the cornstarch to the seasoned berries and toss well.
Step 4Remove dough-lined pie plate from the refrigerator and fill with the berries, spreading so the berries evenly fill the pan.
Step 5Cover the pie: Remove the top crust and/or cutouts from the refrigerator and fit over the prepared pie. Crimp the top crust with the bottom crust if using. If desired, brush the top of the pie with egg wash, and sprinkle over sugar.
Step 6Put the pie on the lowest rack of oven and cook for 20 minutes. After 20 minutes, move pie to a rack in the middle of the oven and reduce the heat to 375 degrees. Keep an eye on the pie; if it begins to brown too quickly, loosely tent with foil.
Step 7Continue to bake until the crust is a deep golden color and the filling is bubbling throughout and the juices have thickened, about 25 minutes more. Carefully remove and cool on a rack until ready to serve. The filling will continue to thicken as the pie cools.
If baking a “slab” pie, the amount of filling and dough will need to be increased depending on the size of the pan (we used double the filling and dough to bake a slab pie using an 18-inch by 13-inch rimmed baking sheet).