Preheat oven to 180°C (160°C fan-forced), lightly grease a 12cm x 36cm rectangle tart tin with removable base and a baking tray lined with baking paper.
Place pastry on a lightly floured kitchen work surface. To check if the pastry is the correct size for the tart tin, place the tart tin over the pastry, there should be at least a 2 ½ cm border the whole way around.
Carefully line the cake tin, prick the base all over with a fork, cover and chill in the refrigerator for 30 minutes.
To prepare the rhubarb cut rhubarb into 10cm lengths. Place in a shallow roasting pan. In a small saucepan combine sugar, orange juice and ginger together over a medium heat, stirring until sugar has dissolved. Pour over the rhubarb, cover with foil and cook in preheated oven for 15 minutes.
Remove foil and cook for a further 15 minutes or until rhubarb is tender, but still holding its shape. Remove from oven and set aside to cool to room temperature.
For the crumble, combine flour, sugar and cinnamon together in a small bowl, mixing together until well combined. Rub the butter into the mixture until large lumps form. Spread crumble onto prepared baking tray.
To blind bake the pastry case, line the pastry case with baking paper. Fill to the top of the tart with baking weights or a dried pulse, to help support the sides and weigh the base down during cooking.
Place tart tin on a baking tray in preheated oven, along with the prepared crumble and bake for 15 minutes. Remove tart tin from oven, check pastry is lightly golden before removing beans and baking paper, using a pastry brush coat the base and sides with the beaten egg. Return tart tin to oven and cook for a further 10 minutes or until golden brown. Remove tart and crumble from oven and allow to cool.
To make the crème pâtissière combine the milk and vanilla bean paste together in a medium saucepan over medium heat. Bring to the boil, remove from heat and allow to infuse for five minutes.
Whisk together the egg yolks, sugar and flours in a medium bowl. Gradually whisk in the hot milk. Place mixture in a clean saucepan over medium heat, stirring continuously until mixture boils and thickens, about 5 minutes. Strain through a fine sieve into a medium bowl, stir in the cream to loosen custartd, cover the surface with plastic wrap and chill in the refrigerator.
To serve, spoon crème pâtissière across the base of the prepared pastry case. Top with rhubarb and drizzle with a little of the pan juices. Sprinkle over the crumble and serve.