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Cut the rhubarb into 10.5cm batons, using pieces that are roughly the same width. Put the vanilla pod, sugar, lemon juice and 300ml water (or enough to just cover the rhubarb) in a wide frying pan or shallow casserole set over a low heat. Once the sugar has dissolved, add the rhubarb and simmer for 5 mins. Remove from the heat and leave to cool in the syrup, preferably overnight, or for at least 1 hr. This way your rhubarb should be perfectly cooked, but still hold its shape.
To make the pastry, put the flour, almonds, sugar and butter in a food processor. Blitz until the mixture resembles breadcrumbs. While the motor is running, add the egg yolk and dribble in 1-2 tbsp cold water. Tip onto a work surface and knead briefly to bring the mixture together to form a dough. Wrap in cling film and chill for 30 mins.
While your pastry chills, make the crème patisserie. Heat the milk and reserved vanilla seeds in a pan set over a medium heat until nearly boiling. Meanwhile, whisk the egg yolks, sugar and flours together in a large bowl until pale. Continue whisking while you pour the hot milk over the egg mixture. Wipe out the saucepan and strain the liquid back into the pan through a sieve. Set over a medium-low heat and stir continuously until the mixture has a thick custard consistency. Scrape into a clean bowl, cover the surface with cling film to prevent a skin from forming and chill for at least 1 hr or up to 2 days.
Remove the pastry from the fridge. If it is a little hard, leave it at room temperature to soften for 10 mins or so. Roll out to 1mm thick on a lightly floured surface. Use the pastry to line a 12 x 35cm fluted rectangular tart tin, making sure you press the pastry into the fluted edges (you can use the blunt end of a knife to help you do this). Leave any excess pastry overhanging. Chill for 30 mins.
Heat oven to 200C/180C fan/gas 6. Remove the pastry case from the fridge, line with baking parchment and fill with baking beans. Blind-bake for 20 mins, then remove the parchment and beans, and continue baking for 8 mins more until pale golden and biscuity. Use a sharp serrated knife to trim off the overhanging pastry to give you a clean edge. Cool in the tin.
Remove the rhubarb pieces from the syrup and set aside. Return the syrup to the hob and boil until thick and sticky. Leave to cool slightly. To finish the crème patisserie, whisk the cream until it holds soft peaks and fold this into the chilled mixture. This is easier if you start by beating in a little cream and then folding in the remaining.
Remove the pastry case from the tin and put on a plate. Fill with the crème patisserie and smooth over the surface. Carefully line up the rhubarb down the length of the tart case, trying to fit them in quite snugly. Use a pastry brush to glaze the tart with the rhubarb syrup. Chill for 30 mins before serving. The tart will keep chilled for three days in the fridge, however it is best eaten on the day it is made.