PREHEAT oven to 375°F (190°C).
CUT rhubarb into 7” (17 cm) lengths. Set aside. Split vanilla bean and scrape out all of the sticky seeds, reserve the pod. Combine the seeds with sugar and lemon juice in a large, wide skillet. Heat over medium -high heat until sugar dissolves, stirring constantly. Lower heat to medium, add rhubarb and cook in sugar syrup for 5 minutes or until fork-tender, not soft. Remove from heat and cool completely.
ROLL pastry on a lightly floured surface into a 9×12” (23×30 cm) rectangle. Gently transfer the pastry into an 8×11” (20×28 cm) rectangular tart pan and gently press pastry into the pan. Using a fork, prick the entire pastry, including the sides. Sprinkle ground almonds over pastry. Line the pastry with parchment paper and fill with dried beans or rice for blind baking.
BAKE in preheated oven for 15 minutes. Remove beans and parchment paper and continue to bake for 5-10 minutes or until golden. Remove from oven and cool completely.
HEAT milk and reserved vanilla pod in a small saucepan over medium-high heat until almost boiling. Whisk egg yolks, sugar, flour and cornstarch together in a bowl. Slowly whisk warm milk mixture into egg mixture until combined. Pour back into saucepan through a strainer to remove any lumps. Whisk constantly over medium-low until mixture thickens to custard consistency. Transfer to a clean bowl, cover with plastic wrap and chill completely.
REMOVE rhubarb from syrup and set aside. Place syrup in saucepan over medium heat and cook until slightly sticky. Remove from heat and set aside.
WHIP cream until soft peaks form. Gently fold into chilled pastry cream.
ASSEMBLE tart by filling with cream mixture. Lay rhubarb in lengths across the cream filling. Brush the rhubarb with the sugar syrup. Slice and serve.