Preheat the oven to 400F. Line a large baking sheet with parchment paper, and lightly dust the parchment paper with flour.
When the dough has chilled for at least 30 minutes, remove one disk at a time from the fridge, remove plastic wrap, and place on the center of the parchment paper.
Roll each disk to about 1/4-inch thick. Keep a plate in the fridge to hold the dough once it's rolled. Once all are rolled, transfer them to the parchment paper-lined baking sheet.
Distribute the pistachio filling in a small circle at the center of each disk of dough, about 2 tablespoons each, leaving a 2- to 3-inch perimeter. Reserve extra for garnish.
Layer the rhubarb on top of the pistachios, about 9 slices per galette. Sprinkle generously with sugar to coat the rhubarb.
Fold in the edges of the dough, layering and pinching sections together to seal any gaps (and prevent leaking).
Brush the exposed dough with the egg wash. Repeat for each galette.
Place the baking sheet on middle rack, and bake for 25-27 minutes (rotating the baking sheet halfway through, and checking at 20 minutes to estimate the remaining baking time), until the rhubarb tips are slightly brulĂȘed, the pastry is golden brown, and the bottom is starting to caramelize.
While the galettes bake, place the leftover pistachio filling in a small frying pan and toast over medium heat until golden brown, stirring continuously, about 3-5 minutes. Remove from heat and set aside to cool, then break up into small clusters.
Remove galettes once done according to instructions above. Transfer to a cooling rack, and let cool completely.