Preheat the oven to 375°F and prepare a baking sheet with parchment paper. In a large bowl combine all the filling ingredients and gently stir to combine.
Place the chilled dough on a lightly floured surface and roll it out with a rolling pin. Start in the middle and work out and get to the desired thickness and size (about 1/4' thick and 10-12" in diameter). Be careful not to stretch the dough. You just need a rough circle or rectangle so don't be too picky. Remember, galettes are "rustic"!
Place the rolled-out dough onto the prepared baking sheet. Gently spoon the mango filling in the middle, leaving 1-2" of dough all around the edges. Fold the edges of the dough over the filling all the way around. Don't worry about being perfect or even, just make sure all the filling stays put. Whisk the egg and brush it on the crust edges. Sprinkle the egg wash generously with brown sugar.
Bake for 45-50 minutes or until the filling is bubbling and the crust is golden brown. I enjoy a slightly burnt sugar crust but you can pull the galette out before the sugar it gets too dark. Allow the galette to cool before serving, but this is best served warm. Enjoy!