To prepare the pastry:

1. Combine flour, salt and sugar in a medium sized bowl.
2. Cut in the butter and shortening with a pastry cutter or the back of a fork until it is the consistency of a course meal with small, pea-sized chunks of butter throughout. Add water, 1-2 tablespoons at a time, tossing gently until pastry comes together in moist clumps. Pat the dough into a round, flat disk. Wrap with Saran wrap and refrigerate for at least an hour.

To prepare the filling:

1. Combine the blueberries, lemon zest, lemon juice, sugar, cornstarch, and salt in a small saucepan. Place the pan over medium heat and stir regularly to break up the blueberries until the mixture is thick and bubbly, about 5 minutes. Spoon into a heat-safe bowl and place a sheet of plastic wrap directly on top of the jam. Allow to cool at least to room temperature.

To prepare the pop-tarts:

1. Whisk an egg in a small bowl for the egg wash and set aside.
2. Roll out the disk of pastry to 1/8” thickness on a floured surface. Using a 2-1/2″ biscuit cutter, gently cut rounds of dough. Each Pop-Tart will require two rounds (one for top and one for bottom). Use a pastry brush to paint the egg wash around the perimeter of half of the circles. Place one to one and a half teaspoons of cooled filling in the center of the rounds with the egg wash. Top the filled rounds with a second circle of crust and use a fork to crimp the edges. Freeze the pop tarts on the baking sheet for 2 hours. Refrigerate the whisked egg in the meantime.
3. When ready to bake, preheat the oven to 375 degrees. Vent the top of each rectangle by poking the top of the pastry with a fork 1-2 times and brush with the remaining egg wash, if desired. Bake the tarts, uncovered, for almost 25 minutes or until golden brown. Allow to cool before topping with glaze.

To prepare the glaze:

1. Whisk together the powdered sugar and 2 tablespoons of juice until smooth. You can add an additional tablespoon of juice if desired, but be sure to keep the glaze thick enough to stay on the pop-tart. You can test how it spreads on the back of a large spoon or try it out on a single pop-tart before you glaze all of them.