Put the leeks, butter and herbs in a pan, cover and cook over a low heat for about 20 mins until very soft, stirring occasionally. While the leeks are cooking, put the potatoes in a pan of cold water, bring to the boil and simmer for 4-5 mins until just cooked. Drain the potatoes and stir into the cooked leeks. Leave to cool, stir in the cheese
and season with plenty of pepper and salt if
you want. The filling can now be chilled for use
the following day, if you like.
Heat oven to 200C/fan 180C/gas 6. Divide the pastry in two and roll one of the pieces to the size of a dinner plate. Transfer this to a baking sheet and roll the remaining pastry and any trimmings to a round about 5cm bigger than the first. Pile the filling into the middle of the round on the baking sheet, leaving a 4cm border. Brush the border with the beaten egg, then drape over the larger piece of pastry. Trim the edges to neaten,
then press the sides together with your thumb. Brush the tart all over with egg. Bake for 35-40 mins until golden. Leave to rest for 10 mins before cutting into wedges and serving with beans or greens.