In a bowl, combine the flour, salt, and 2 tablespoons sugar.
Using your hands, work in the cold butter until it resembles coarse sand. It's okay to have some bigger chunks.
Beat the egg and mix it into the dough.
Transfer the dough to a flat surface and quickly shape it into a rough disk that's about 1-inch thick.
Wrap the dough in plastic and place it in the freezer for 20–30 minutes, until firm.
Meanwhile, make the jam by combining the fresh blueberries, ⅓ cup sugar, 1 tablespoon lemon juice, and 2 tablespoons water in a saucepan.
Cook on medium heat, stirring often, until thickened, about 15 minutes. Set aside to cool.
Preheat the oven to 350 F.
Line a baking sheet with parchment paper.
Roll the chilled dough on a floured surface, forming a 14x12-inch rectangle that's about ⅛-inch thick.
Cut the dough into 16 even rectangles.
Fill 8 of the squares with 1 tablespoon cream cheese, then add 1 tablespoon of the blueberry jam.
Put the other pieces of dough on top and seal the ends with a fork.
Place in the freezer for about 15 minutes.
Bake the Pop-Tarts on the baking tray on the middle rack for 20–25 minutes, or until the edges are golden brown and the center is slightly golden.
While the Pop-Tarts are baking, make the glaze by combining the powdered sugar with 1 tablespoon lemon juice and 1 tablespoon water until smooth but not too runny. Add a few more drops of lemon juice or water if it's still thick.
Spread about 2 tablespoons of the glaze on the top of each Pop-Tart.