Blueberry filling:
In a medium saucepan, combine blueberries, sugar and salt. Add lemon juice and cornstarch; toss to combine. Cook over medium heat, stirring constantly (but gently), until blueberries release their juices and the mixture comes to a boil. Boil for about 30
seconds, until thickened (the mixture should be thick). Transfer to a bowl and cool completely, then refrigerate for at least 30 minutes.

The dough:
Using the paddle attachment or beaters, in a stand mixer combine the flours and salt at low speed. Add the butter pieces and mix for about a minute. The mixture will be very crumbly, with large pieces of butter in it. Continue mixing, adding the ice cold water, until the dough starts to come together. Place dough on floured surface and roll out into a rectangle. Roll the dough into a ball and roll it out again; repeat two more times, lightly dusting the dough with flour as you go. Fold final rectangle dough in half, then in half and in half again. Wrap in plastic wrap and refrigerate for at least 30 minutes.
Unfold dough and cut in half to make two rectangles. Refrig-erate one while working with the other. Lightly dust surface with flour. Roll out the rectangle to 1/4" thickness and trim to a perfect 12-inch square. Cut into 4 smaller squares, being careful to make them even. Spoon 1/4 cup of filling on half of the square and fold other half diagonally over it, pressing the edges to seal. Press down edges using the tines of a fork. Refrigerate turnovers as you finish them, chilling them for at least 30 minutes before baking. Repeat with the remaining dough.
Preheat oven to 400°F. Line a baking sheet with parchment paper. Arrange the flips on a baking sheet and brush them with the egg wash. Cut 2 or 3 slits in the top of each turnover, then sprinkle with coarse sugar. Bake for 25-30 minutes, until golden brown. Serve warm or reheat later.
(To make smaller round pastries, roll out dough to 1/4” thickness and cut out 1 1/2” circles. Place 2 tbsp filling in centre of one circle, top with another circle and pinch edges together, sealing with a fork. Brush edges with eggwash to seal. Cut an X in the top of the dough, sprinkle with sugar and bake for 20-25 min. until golden.)