Preheat the oven to 425ºF.
Remove the dough from the refrigerator and unfold it onto a well-floured surface, rolling it out to a 14x14-inch square. (See Kelly's Notes.)
Using a 3-inch circular cookie cutter or cup, cut out as many circles as possible. Re-roll the scraps and continue cutting out circles until you run out of dough, ensuring you end up with an even number of circles. (You should have approximately 20 3-inch circles.)
Place six of the dough circles on a Silpat or parchment paper-lined cookie sheet. Spread a portion of the Nutella in the centers of half of the dough circles leaving a 1/4-inch border around the edges. Spoon a small portion of the strawberries atop the Nutella. Place a second dough circle atop each filled circle then use a fork to crimp the edges together, sealing each pie.
Whisk together the egg with the water to make an egg wash. Brush each pie with the egg wash, and using a sharp knife, cut two or three vents on the top of each pie. Sprinkle the pies with crystal sanding sugar (optional).
Bake the pies for about 15 minutes, or until they're golden brown.
Remove the pies from the oven and allow them to rest for 10 minutes before serving. Repeat the filling and baking process with the remaining dough circles.