potato, cheese and onion pie

from John Waite (via Gail) - a substantial, tasty pie suitable for a vegetarian Christmas, or any time for comfort food!

for 6-8
You will need a 23cm dia pie dish

Pastry:
225g plain flour, plus extra for dusting
1 tsp fine sea salt
150g unsalted butter, cubed
75ml whole milk
1 tsp cider vinegar
1 egg yolk, to glaze

Filling:
750g Maris Piper potatoes
1 medium onion, finely chopped
300ml crème fraîche
1 tsp wholegrain mustard
3 salad onions, finely sliced
25g pack parsley, roughly chopped
3 tbsp finely chopped chives
250g mature Cheddar, grated
200g Gruyère, grated
Finely ground white pepper, to season

Pastry:

• Put the flour, salt and butter into a bowl and rub in until you have 'breadcrumbs'. 
• Make a well in the centre. In a small jug mix together the milk and vinegar, then add it all to the dry ingredients. 
• Using a blunt knife, cut the milk into the flour until it starts to clump together. When it does (and most of the dry, powdery bits of flour are incorporated) knead the pastry briefly – no more than 20 seconds – until it is reasonably smooth. 
• Flatten into a disc, wrap in clingfilm and chill for at least 30 minutes.

Filling:

• Wash and quarter the potatoes – unpeeled. 
• Steam the potatoes for 25 minutes, or until very easily pierced with a knife. 
• Remove them from the steamer, then return them to the hot, dry pan. Add the onion and leave to steam for 5 minutes more. 
• Add the remaining filling ingredients, stirring to combine, and seasoning to taste with salt and white pepper – some of the potatoes will fall apart, but don’t worry, just get it all evenly combined.

Assembling:

• Preheat the oven to 200°C and place a baking sheet in the oven to heat up.
• Take two thirds of the pastry and roll it out on a well-floured worktop, until it forms a disc big enough to line the base of the pie dish, with surplus hanging over the sides. 
• Pile the cooled potato mixture into the pie dish and level off. 
• Roll out the remaining pastry to a disc big enough to top the pie, again with a little excess, and place it on top of the potatoes. 
• With a sharp knife, trim away the excess pastry, then crimp the pastry together at the edges.
  Beat the egg yolk with a pinch of salt and a drop of water and use it to glaze the top of the pie. 
• Cut a hole in the centre to let out steam. 
• Place onto the hot baking sheet in the oven and bake for 30-35 minutes, until the pie is a deep golden brown. 
• Allow to rest for 5 minutes or so before serving – this will help to set the filling.