Cut the rhubarb into bite-size pieces, then put them in a frying pan with half the sugar and warm through just until the sugar dissolves. Immediately tip the rhubarb into a bowl with the juices and leave to cool. Preheat the oven to 180C/gas 4/fan 160C.
Roll out the pastry quite thinly and use to line a deep, loose-based 24cm fluted flan tin. Line with greaseproof paper and baking beans and bake for about 20 minutes until the pastry is pale golden and no longer raw.
Beat together the egg and egg yolk, vanilla extract, remaining caster sugar and the flour. Gradually whisk in the cream with any juice from the rhubarb (you should have a tablespoonful or two). Now spoon the rhubarb into the prepared pastry case and pour the cream mixture over.
Turn the oven temperature up to 200C/gas 6/ fan 180C and bake for about 20 minutes, or until the custard is very lightly set and there is a thin skin on the top.
Mix together all the topping ingredients and spoon evenly over the pie. Return it to the oven for a further 15 minutes or until the crumble is golden and the custard set with just a little wobble. The top of the pie may have risen and cracked when you take it from the oven but dont worry as it will settle back again on cooling.
