Prepare the Roasted rhubarb and leave to cool. Turn oven to 190C/fan 170C/gas 5. Roll the pastry out on a lightly floured surface to the thickness of a £1 coin and use it to line a 25cm flan dish, leaving some pastry hanging over the edge. Prick the base of the pastry lightly with a fork, then line with greaseproof paper. Tip some baking or dried beans into the pastry case, put it on a baking sheet and bake for 20 mins until the pastry edges become biscuity. Remove the beans and cook the pastry for a further 5 mins until brown; you want to cook it a little more than you would normally. Remove the pastry case from the oven and reduce the temperature to 160C/fan 140C/gas 3.
While the pastry is baking, make the custard. Put the cream, milk and vanilla in a pan, bring to the boil, then take off the heat. Beat the sugar and eggs to combine, then pour in the hot liquid while whisking continuously. Strain the custard mixture into a jug.
Scatter the rhubarb over the bottom of the pastry case, put the case in the oven and carefully pour in the custard. Bake the tart for 30-35 mins until the custard is just set. Leave it to cool in the dish, then trim off the excess pastry with a knife before serving.