Preheat oven to 375°F. Unroll puff pastry. Add parchment paper (from puff pastry) to baking tray.
On a well floured surface, roll puff pastry to 14” (square or round, shape is not important) and place puff pastry on the parchment lined baking tray. Pop back into refrigerator while you prepare the rhubarb.
In a medium bowl, mix ½ cup sugar and ginger together (to help the ginger distribute well). Stir in rhubarb, butter, cornstarch, and vanilla.
Place rhubarb mixture in the center of the puff pastry, leaving a 1 ½ to 2 inch border. Fold pastry over top of rhubarb, turning as you go. Pastry will overlap.
Beat egg, with salt in using. Brush pastry with beaten egg and sprinkle with 1 teaspoon sugar.
Bake for 25-30 minutes or until pastry is gold and rhubarb is cooked. Cool 5 minutes before cutting and serving.