First, make the pastry by combining flour and salt in a food processor. Next add in cubed, cold butter and pulse until small pebbles have formed. Slowly add in cold water until the dough is combined.
Roll the pastry dough into a ball, flatten it to a disc, and place it in a ziploc bag. Refrigerate the dough for at least an hour, but preferably overnight.
Make the blueberry compote by combining blueberries, cornstarch, sugar, and lemon juice in a saucepan. Cook over medium heat until the sauce starts to thicken (about 10 minutes), stirring occasionally. The sauce will thicken more as it cools.
To make the blueberry lemon pop tarts, cut the dough in half and place the second half back in the refrigerator. Roll out the pastry dough on a lightly floured surface until about 13 inches by 11 inches and 1/4 inch thick. Trim the sides so that they are even and straight and then cut the dough into eight 5 by 3 inch rectangles.
Arrange four rectangles onto a parchment-lined baking sheet and fill with one tablespoon of the blueberry compote. Make sure to leave a 1/2-inch border around the outside.
Place the second rectangle on top and use a fork to crimp the edges of the pop tart. Using a fork, poke a few holes on the top of each pop tart to allow steam to escape while baking.
Repeat with the second half of the pastry dough. Then place the baking sheets in the fridge and refrigerate for 30 minutes.
Make an egg wash and brush each pop tart with the egg wash. Bake the pop tarts at 375 degrees F for 20 minutes or until golden brown. Cool before icing.
To make the lemon icing combine powdered sugar and lemon juice. You may need to add more powdered sugar or lemon juice until you reach the desired consistency. Then spread the icing on the pop tarts. Optional: sprinkle lemon zest on top of the icing.