In a mixing bowl, combine flour, sugar, and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

In a large bowl, combine chopped rhubarb, sugar, cornstarch, vanilla extract, and lemon juice. Toss until rhubarb is well coated.

Preheat the oven to 375°F (190°C). Roll out the chilled pastry on a floured surface into a 12-inch circle.

Transfer the pastry to a baking sheet lined with parchment paper. Spoon the rhubarb filling into the center, leaving a 2-inch border.

Fold the edges of the pastry over the filling, pleating as necessary. Brush the crust with beaten egg and sprinkle with sugar.

Bake in the preheated oven for 35-40 minutes, or until the crust is golden brown and the filling is bubbling.

Allow to cool slightly before serving.