Cut the rhubarb into 8cm pieces so that it fits a rectangular tart dish.
Add the water, vanilla pod (seeds removed and reserved for later), caster sugar and lemon juice to a medium sized saucepan. Heat over low heat until the sugar is dissolved, increase the heat and add the rhubarb. Simmer for about 5-6 minutes. Then, remove from the heat and leave to cool in the infused water for 1 hour.
For the pastry, place the plain flour, ground almonds, icing sugar, butter and egg yolk in the TM bowl. Mix 20 sec/speed 6. While mixing, add a splash of cold water until the dough comes together.
Wrap the pastry in clingfilm and chill for 30 minutes. Meanwhile, to make the creme patisserie, place the milk, egg yolks, caster sugar, pinch of salt, corn flour and reserved vanilla pod seeds in the TM bowl. Cook 9 min/90°C/speed 3.
Remove the creme patisserie from the mixing bowl and place on a shallow dish. Cover with clingfilm and leave to cool entirely. Preheat the oven to 200°C.
Remove the pastry from the fridge and uncover. Place on a floured surface and roll out to fit the rectangular tart dish.
Place greaseproof paper on the tart shell and cover with baking beans. Blind bake in the oven for 15 minutes, then remove the baking beans and greaseproof paper and brown for another 5-8 minutes.
Leave the tart to cool entirely. Meanwhile, remove the rhubarb from the syrup and place aside. Bring the syrup back to boil for another 10 minutes until reduced.
To finish off the creme patisserie, take the whipped cream and fold under the cold custard until incorporated. Spread over the cooled tart base and decorate the rhubarb on top. Then brush over the syrup and enjoy!