In a mixing bowl, combine all-purpose flour and granulated sugar. Cut in the cold unsalted butter until the mixture resembles coarse crumbs. Stir in the egg yolk, then form the dough into a disk. Wrap it in plastic wrap and chill in the refrigerator for about 30 minutes.

While the dough is chilling, preheat your oven to 400°F (200°C). Prepare your baking sheet by lining it with parchment paper.

In a large bowl, combine the chopped fresh rhubarb, sugar, pistachios, and vanilla extract. Mix gently and let the filling sit for about 10-15 minutes.

Once the dough has chilled, place it on a lightly floured surface and roll it out into a roughly 12-inch circle, about ⅛ inch thick. Transfer the rolled dough to the prepared baking sheet.

Spoon the rhubarb filling into the center of the rolled dough, leaving about a two-inch border around the edges. Fold the dough edges towards the center.

Brush the edges of the galette with an egg wash and sprinkle with a little granulated sugar. Bake for 35-40 minutes, or until the crust is golden brown and the filling is bubbly.