Combine flour, salt, butter and egg yolk in a processor until the mixture resembles breadcrumbs. Add grated Black Bomber and combine.
Add water and mix. Roll dough and wrap in cling film. Chill for 30 minutes.
Boil potato pieces for 10-15 minutes until soft. Drain well and steam dry.
Fry sliced onions in butter and thyme until soft. Add 150ml stout and reduce until the liquid has gone and then remove the thyme. Season filling.
Sprinkle the onions with the cornflour and stir well to coat. Add the milk and cream and heat the mixture over a medium heat for 5 minutes, stirring continuously, until the liquid is smooth and has thickened slightly. Add the cooked potato pieces, grated cheese, Dijon mustard and stir well.
Preheat oven to 180°c and grease tin with butter.
Roll out two thirds of the pastry before placing over the tin, lining the base and sides.
Blind bake in the oven for 10-15 minutes, until the pastry is golden-brown. Once cool, add the pie filling.
Roll remaining third of pastry. Brush the rim of the cooked pastry case with egg and place pastry lid on top. Trim off excess pastry and crimp the edges with thumb and forefinger. Make two small holes in the centre of the pastry lid.
Brush pastry lid with remaining egg; oven bake for 25-30 minutes until golden-brown.
Delicious served with a hot cheese sauce or some Snowdonia Caramelised Balsamic Onion Chutney.