Cut up cold butter into small pieces and place in freezer for 5-10 minutes. In a food processor, pulse flour, sugar, and salt until combined. Add butter and pulse until mixture has no large pieces of butter remaining. Add egg and 1 tablespoons ice water, then pulse until well combined and dough holds together when squeezed, 1 minute. Add water one table spoon at a time if needed. Roll into a ball and place in the fridge or freezer for 5-10 minutes.
Preheat oven to 375F. In a large bowl, add washed and dried fresh blueberries, GF flour, and cornstarch and mix. Add maple syrup and lemon juice and mix well.
On a very well floured counter, roll out dough until about 1-8-1/4 inch thick. Place in pie dish. Add pie filling to the center and fold over extra pie crust. In a small bowl, make an egg wash by scrambling one egg and brush over pie crust. Sprinkle with sugar. Bake for 50-60 minutes, until crust is golden brown. Cook and enjoy.