In a large bowl, combine blueberries, sugar, flour, cardamom, lemon zest, and lemon juice. Stir gently with a spoon. Set aside.
Line a large cookie sheet with parchment paper. Unroll the first roll of pie crust.
Pour half the berries into the center of the dough. Fold up one side of the dough 3" over the berries. Place your right hand over the dough fold, then use your left hand to fold up the next corner. Repeat these steps until all the dough has been folder up over the berries.
Move the first galette to the side and then repeat the above steps with the second roll of dough.
Brush each galette with the egg, then sprinkle the sugar over the pastry.
Bake for 25-30 minutes or until golden brown on top. Allow the galettes to cool for 20 minutes before serving.