In a bowl, rub the butter into the flour with your fingers until it resembles fine breadcrumbs.
Mix in the egg with a knife, then knead on a clean, lightly dusted work surface to form a smooth dough.
Use the dough to line a 23cm/9in loose-bottomed tart tin, prick the base all over with a fork and leave to rest in the fridge for about 30 minutes.
Preheat the oven to 190C/375F/Gas 5.
Line the pastry with parchment paper and weigh down with rice or ceramic baking beans. Bake the pastry blind for 10-15 minutes, remove the paper and rice or beans and return the pastry case to the oven for a few minutes more, until light golden-brown.
For the filling, mix together the filling ingredients in a bowl and pour into the pastry case. Return to the oven and bake for about 30 minutes. Serve hot or cold with clotted cream or double cream.