Preheat oven to 400 degrees. Place parchment paper on baking sheet or spray well with cooking spray.
Pop tube! And spread out on baking sheet. Sprinkle entire crust with about 2 tablespoons of the sugar.
Slice apples into thin (1/4 inch thick) slices and add to large bowl.
Add remaining brown sugar, flour, cinnamon and salt to apples and stir well to combine.
Spread apple mixture on top of crust, leaving a 1 1/2" - 2 inch border clear around apples.
Turn in crust on top of apples, forming a rustic, open top pie. Dot apples with butter.
Make egg wash: crack egg into small bowl, add a teaspoon of water and whisk well. Bush crust using a pastry brush or paper towel.
Bake for 35 to 40 minutes until crust is brown and crispy and apples are cooked through. (You might have to test one or two.) After galette cools, dust with confectioner's sugar if desired.