Preheat the oven to 400 degrees F.
For the Sheet Pan - Line a large sheet pan (with sides) with parchment paper.
For the Galette - On the lined sheet pan, unroll the pie dough to form a large circle.
For the Filling - To the middle of the pie dough, add cranberry sauce (whole or jellied). Using the back of a spoon, or a soft spatula, spread the cranberry sauce out to about 3 inches from the edges of the dough.
Sprinkle sugar evenly on top of the cranberry sauce. Top the cranberry sauce with blueberries in an even layer.
Fold in the edges of the pie dough (the edges without any filling on it) toward the middle of the galette to form a "ring" of pastry around the outside edge of the galette.When folding in the edges of the pie dough, crease them a little so that when looking at the galette from a top view, the filling has a circular shape.
Adjust the dough if needed so that the amount of dough folded over the filling is about the same width.
For the Egg Wash - In a small bowl, add egg and water. Whisk together. Brush the egg wash onto the surface of the pie dough
As soon as possible after brushing on the egg wash, add the sanding sugar to the pie crust so that it will stick.
For the Sanding Sugar - Sprinkle sanding sugar (or other large crystal sugar) on top of the pie crust and filling.
Baking the Galette - Bake in a preheated oven (400 degrees F.) until the crust turns a light golden-brown color (about 31 to 33 minutes). Remove the galette from the oven and let it cool a little bit before serving.
Serve warm just as is or topped with vanilla ice cream or whipped cream.