In the bowl of a food processor, add the flour, sugar and salt. Pulse a few times to combine.
Next, dice the cold butter and add it to the food processor. Pulse it about 10 times to evenly incorporate it into the dough--the pieces should be about the size of peas.
Slowly stream in half the water, and pulse dough a few times to bring it together. Stream in remaining half of the water and pulse the dough until it comes together in a ball.
Remove dough from food processor. Divide it in half, wrap each half in plastic wrap, and press it flat into a disk. Chill for 15 min.
Flour a work surface and a rolling pin very well. Roll out a dough disk into a rectangle measuring roughly 15" x 8". Trim edges into a rectangle and cut 6 small rectangles that measure roughly 5 x 3".
Move the 6 rectangles to a baking sheet lined with parchment paper. Repeat with the other dough disk, but these 6 rectangles are your top crusts. Set them aside.
Preheat the oven to 425F and In a small bowl, combine the blueberries, corn starch, sugar, lemon zest and juice, cinnamon and salt. Stir to combine. Set aside.
Next, beat together the egg and milk mixture. Brush it on the bottom crusts evenly.