Press one of the pie crusts evenly into the bottom of an eight-by-eight-inch baking pan.

Put the pan, along with the other crust, into the fridge while you prepare the filling.

Preheat the oven to 375 degrees.

Add the blueberries, brown sugar, lemon juice, cinnamon, allspice and salt in a saucepan over medium heat, stirring regularly until the mixture comes to a boil.

While the mixture simmers, add the cornstarch, along with one tablespoon of water, to a small bowl and stir to dissolve the cornstarch. Add it into the blueberry mixture, and continue to stir often, letting it cook for a few more minutes until the mixture is thickened.

Remove it from the heat, and set it aside.

Bake the bottom pie crust for 20 to 30 minutes or until the edges are golden brown.

Roll out the second crust and cut it into about one-half-inch strips.

Weave the dough into a lattice, placing it on parchment paper and trim the lattice to an eight-by-eight square. Slide the parchment onto a baking sheet and put it in the freezer.

When the bottom crust is done, spread the blueberry filling evenly on top and place the lattice on top.

Use a pastry brush to coat the top of the lattice with the beaten egg, and then sprinkle the top liberally with sugar.

Bake for 25 to 30 minutes or until the top is golden and the filling is bubbling.

Let the bars cool completely before slicing. Store the bars in an airtight container.