Heat a large frying pan, melt the butter,
then add the onion and leeks. Cover, then
very gently cook for 15 mins until soft.
Add the potatoes after 5 mins cooking.
Stir in the flour, turn up the heat, then add
the stock, stirring until the mix thickens a
little. Take off the heat, then stir in the crème
fraîche, mustard and ham. Season to taste,
then cool, if you have time. Spoon the mix
into a large pie dish, adding a couple of bay
leaves. Heat oven to 220C/fan 200C/gas 7.
Roll out the pastry on a floured surface
to the thickness of a £1 coin. Cut a pastry
strip the same width as the lip of the dish,
wet the edge of the dish with egg, then
fix the strip around it. Brush the top with
egg, then lay over the remaining pastry.
Trim pastry to the edge of the dish, then
ruffle the cut edges with the knife blade to
help the layers rise. Cut a few slits to let the
steam escape, decorate with bay leaves, if
you like, then brush all over the top with
egg. Can be done up to a day ahead, keep
chilled. Bake for 30 mins until risen.