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Put the flour, icing sugar and a pinch of salt in a large bowl and mix together. Add the butter and rub together until the mixture resembles breadcrumbs. Add 1 egg yolk, the vanilla and 1 tbsp cold water, and mix together until it just starts to come together as a dough. Tip the mixture onto a clean work surface and gently bring together with your hands. Wrap the pastry in cling film and chill for at least 1 hr before rolling out. Can be made 3 days ahead, or frozen for 2 months.
Put the vanilla beans scraped from the pod (or the paste) in a pan over a medium-high heat, add the milk and bring to the boil. Meanwhile, tip the egg, yolks, sugar and cornflour into a bowl and whisk together until smooth. Pour the milk over the egg mixture, whisking to combine. Pour the custard back into the pan and cook, whisking constantly, for 2-3 mins until thickened. Scrape into a bowl and add the butter, mixing until melted and combined. Press a sheet of cling film onto the surface of the custard to stop a skin forming, and chill for 3 hrs. Can be made and chilled 3 days ahead.
On a lightly floured surface, roll out the pastry into a large rectangle big enough to line a 30 x 20cm fluted rectangular tart tin. Roll the pastry onto the rolling pin and carefully drape it into the tin, carefully lifting and pressing into the corners and edges. Roll your rolling pin over the tart tin, cutting off the excess. Chill for 30 mins or until the pastry is firm.
Heat oven to 180C/160C fan/gas 4. Line the tart with a piece of crumpled baking parchment and fill with baking beans or rice and place on a baking tray. Bake for about 30 mins, then remove the parchment and the beans, and return to the oven for another 5 mins or until the base is golden brown. Brush the inside of the tart with the remaining beaten egg yolk and return to the oven for 1 min to set (this creates a seal, meaning the pastry won’t become soggy as quickly). Set aside to cool.
Heat oven to 190C/170C fan/gas 5. Put the rhubarb batons in a small roasting tin (so that they are in one flat layer) and sprinkle over the sugar, the vanilla pod and its scraped out seeds, and the orange juice. Roast for 15-20 mins, or until the rhubarb has softened but is still holding its shape and a vibrant pink syrup has formed. Remove from the oven, discard the vanilla pod and allow to cool.
Remove the custard from the fridge, beat to loosen, then pour over the pastry and smooth with a spatula. Top with the roasted rhubarb, brushing a little of the syrup on top, then sprinkle over the pistachios. Best eaten on the day its made.