A layer of robust, jammy rhubarb in this tart highlights the warm vanilla bean. The gorgeous herringbone design makes it an ideal dessert for special occasions and parties.
Preheat oven to 375 degrees. Line a baking sheet with parchment. On a lightly floured surface, roll out dough to a 10-by-13-inch rectangle, about 1/8 inch thick. Transfer to prepared sheet and refrigerate 15 minutes.
Meanwhile, whisk together granulated sugar, cornstarch, and salt. Using your fingers, work vanilla seeds through mixture evenly. Add rhubarb and lemon juice; stir to combine.
Arrange rhubarb mixture over dough, in a herringbone pattern if desired, leaving a 1 inch border. Fold edges of dough over to partially cover rhubarb. Brush crust with egg, sprinkle with sanding sugar, and bake until bubbling in center, 45 to 50 minutes.
Using a pastry brush, spread bubbling juices over rhubarb to glaze. Let cool at least 30 minutes. Tart can be stored, tented with foil , at room temperature up to 1 day.