Preheat your oven to 400F and prepare a baking sheet lined with parchment paper.
In a large bowl, mix 2 cups quartered strawberries, 1 ½ cups sliced rhubarb, ½ cup sugar, 1 teaspoon vanilla extract, 1 tablespoon lemon zest and 2 tablespoons cornstarch. Stir gently to ensure that the strawberries and rhubarb don't turn to mush. Set aside.
Turn your pie dough onto a lightly floured surface and roll out into a 12 inch circle (about ⅛ inch thick). Carefully transfer dough to your baking sheet.
Spoon filling into the middle of the dough ensuring you leave a 2 inch border all around (be careful not to spoon too much of the juice else your galette will be very runny).
Fold the dough over the filling and slightly pinch where the overlaps occur. See video for guide on how to fold your galette.
Dot the filling with butter and brush the edges of the galette with egg wash. Sprinkle demerara sugar and bake until the fruit filling is bubbling and the crust is golden brown (about 45 minutes).
Serve galette warm. Optionally, you can sprinkle with confectioner's sugar or top with vanilla ice cream.
You may need to add more sugar depending on how sweet or tart you want your galette to be. ½ cup results to a galette that's more on the tart side.