Make the pastry: In a food processor mix the flour and sugar. Add the cold cubed butter and blitz together until it resembles fine breadcrumbs. Continue to pulse, until the mixture comes together to form the pastry. Take 6x 10cm individual pastry tins and press the pastry into each, lining the base and sides. Gently prick the bottom with a fork and then cover with cling film and place in the freezer to allow the pastry cases to chill for 30 minutes.
Remove from the freezer and place them in on a baking tray in the middle of the oven for 15 - 20 minutes until lightly golden. Remove from the oven and allow them to cool completely. Turn the oven down to 180C.
Make the creme patisserie: In a large bowl whisk together the egg yolks, sugar and flour. Place the milk in a saucepan and scrape in the vanilla pod seeds, then place on the heat and bring to scalding point - small bubbles with begin to show around the edges and the milk will begin to steam. Remove from the heat and slowly pour into the egg mixture, whisking whilst pouring. Once all the milk has been whisked in, wash out the pan and then pour the mixture back in. Place it back on the heat and gently heat, stirring continuously until thickened. Once thickened and smooth remove from the heat and place in a bowl with cling film closely covered and allow to cool in the fridge.
Roast the rhubarb: Decide the patterns you wish to decorate with the rhubarb pieces and cut them accordingly. Place all the cut pieces of rhubarb into a small roasting tray with the orange juice, vanilla pod, and sugar. Mix well so everything is well coated. Place in the oven and roast for 7 - 10 minutes until softened but the rhubarb still holds its shape. Remove from the oven and allow it to cool.
Assemble: Fill each pastry case with the creme patisserie and then decorate on top with the pieces of rhubarb in a pattern. Finally decorate with some finely chopped pistachios and serve immediately.