Use your finger to brush the edges with a light coat of water and fold the edges of all 4 sides over to create a lip. Whisk together the egg and water, to create an egg wash. Use a fork to poke holes all over the center of the pastry, and then brush with a thin layer of the egg wash. Sprinkle a teaspoon or so of sugar over the entire bottom, and bake for 15-20 minutes, until the crust is golden brown. Remove from oven and gently press down the center of the pastry that has puffed up, to flatten it out. Transfer to a wire rack to cool.