Whisk together the flour, salt, baking powder, and sugar.
Add the butter, working it in with your hands to make a coarse meal mixture. The butter should still be in large, pea-sized pieces.
Stir in the Greek yogurt, poppy seeds, and lemon zest; the dough will be shaggy. Turn it out onto a well floured work surface, and bring it together with a few quick kneads.
Pat the dough into a 10 inch log, then roll it out into a 10" x 12" rectangle. Dust both sides of the dough lightly with flour, and starting with a shorter end, fold it in three - like a letter.
Flip the dough over, and roll it again into a 10" x 12" rectangle. Fold it in three again.
Wrap the dough in saran wrap, and chill for 45 minutes.
To make the blueberry filling:
Combine all the ingredients in a saucepan set over medium heat. Cook until the mixture starts to thicken, about 6 minutes. Transfer the berries to a bowl and let cool to room temperature.
Assembly and cooking:
Preheat the oven to 425°(F). place a rack on the middle shelf. Line a large baking sheet with parchment paper.
Roll the dough into a large square, about 16" x 16". Using a pastry wheel cut out sixteen even squares. Don't worry if they're not exactly the same size. Just do the best you can ;)
Divide the cooled filling among eight of the squares, using about a tablespoon and a half for each. Brush a little bit of the beaten egg wash along the edges of each filled square.
Cut a vent (or heart!) into each of the remaining eight squares, using a decorative cutter, small knife, or bottle cap! You can really get creative here.
Top each filled square with a vented square, and press along the edges with the front of a fork or a pie crust crimper; seal well!
Brush the top of each pie with the remaining beaten egg, and sprinkle with sugar.
Transfer the pies to prepared baking sheet.
Bake the pies for 18 to 20 minutes. Remove them from the oven, and let cool for 20 minutes before serving.
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