Preheat the oven to 375 degrees. Line a rimmed baking sheet with parchment paper.
Combine the strawberries, sugar, lemon juice, strawberry preserves, vanilla, and cornstarch in a medium bowl. Mix gently to combine.
Roll out the dough onto a floured surface to a circle about ¼ inch thick and 14 inches in diameter. Place the crust onto the baking sheet.
Pour the filling onto the crust, leaving about 2 inches at the edge. Fold the edge of the crust up over the strawberries all the way around the galette, pleating the dough as necessary to form a circle.
Beat the egg with 1 teaspoon of water and brush the edges of the pastry with the egg wash. Sprinkle coarse sugar over the egg wash.
Place in the preheated oven and bake until the filling is bubbly and crust is golden brown, about 35-40 minutes. The galette will leak some during cooking. Allow to cool about 15 minutes before slicing.