In a medium-sized bowl, combine all the ingredients for the blueberry filling and place in the fridge
For your pastry dough, combine all your dry ingredients in a large bowl and whisk until uniform.
Add sliced cold butter into dry ingredients, and using your fingertips, press each slice of butter into the flour until the butter is pea-sized and the flour looks crumbly. (Heat is the enemy here, so refrigerate when needed)
Add ice-cold water and mix until just combined. Cover with plastic wrap and place in the fridge while your oven preheats to 400°F.
In a well-dusted, clean tabletop, cut your dough in half (save the other half for another time). Shape your half dough into a ball and roll out into a circle until it’s about ⅛ inch.
Place your Blueberry Filling into the middle and spread, making sure to leave about a 3-inch space from the sides.
Fold up the sides of the dough and pleat.
Optional: brush the ready-to-bake dough with egg wash (1 egg + 1 tablespoon water) and sprinkle on an extra tbsp of sugar for a shiny, slightly crispy top crust!
Bake in the middle of a 400°F oven for 30-35 minutes or until the pastry is golden brown!
Allow to rest on a cooling rack for 20 minutes before serving.