Pre-heat the oven to 175°C.
For the pastry, mix together the flour and sugar, then with your fingertips rub in the salt and butter until the mixture resembles breadcrumbs.
Beat the whole egg and slowly add to the breadcrumb mixture until the pastry forms a ball. Wrap tightly in cling film and refrigerate for two hours.
Remove the pastry from the cling film and roll out onto a lightly floured surface to 1/4cm in thickness. Line an 27cm ring with the pastry and place it onto a baking sheet. Place the lined tart ring in the fridge for 20 minutes.
Line the pastry with greaseproof paper and fill with baking beans to keep the base's shape, and blind bake for about 20 minutes. Remove the baking beans and greaseproof paper and return to the oven for a further 5-8 minutes, or until the pastry is starting to turn golden brown. Remove from the oven, brush with egg yolk and allow to cool.
Turn the oven down to 125°C.
For the filling, pour the cream into a saucepan. Split the vanilla pods and place them in the cream. Bring the cream to the boil. Take off the heat and allow to cool for a few minutes. Whisk together the yolks and sugar. Whisk in the cream and mix well. Pass the mixture through a sieve into a jug.
Fill the pastry case with the custard. Carefully place in the middle of the oven and bake for anything between 40-50 minutes, or until the custard appears set. Remove from the oven and allow to cool in the fridge.
Turn the oven up to 175°C.
Cut the rhubarb into neat 1 inch slices. Place the rhubarb on a baking tray lined with greaseproof paper and sprinkle with the caster sugar. It's up to you how sweet you want your rhubarb, I prefer mine quite sour to go with the sweetness of the custard.
Bake in the oven for around 10-12 minutes or until the rhubarb starts to go soft. Do not allow to caramelize. Allow to cool.
Carefully take the rhubarb and place open side up on the tart starting from the outside edge, working towards the middle, until you have a mosaic type filled tart.