Step 1

To make the pastry, tip the flours into a bowl and mix with the salt. Tip in the butter and squash it into the flour with your fingers, rubbing it together. Stop when you have a mostly breadcrumb texture with the odd lump of butter. Mix the vinegar with 2-3 tbsp of ice-cold water and drizzle over the flour mix, then use a knife to stir in a figure of 8 movement until it starts to clump. Bring together briefly with your hands. Wrap in clingfilm and chill in the fridge for 30 minutes.

Step 2

Halve and peel each onion, then take off the first layer (you can freeze this for stock). If they are really old onions, take off the second layer as well – the onions need to melt into the sauce and the outer layers of old onions will be too tough to break down. You want to end up with around 450-500g of peeled onions. Chop the onions and add to a non-stick pan with 200ml water and the butter. Put on a lid and simmer for 20 minutes, then take off the lid, turn down the heat and cook until most of the moisture has gone and you have an onion purée that is gently frying in the butter but not colouring (this can take 20-30 minutes, so be patient). Take off the heat, season and cool.

Step 3

Drop the potatoes into boiling salted water and cook for around 5 minutes, until really soft. They should be really tender but holding their shape. Drain and leave to cool.

Step 4

Lightly flour the worksurface, then rollout 2/3 of the pastry to line a deep 20cm pie tin, keeping the pastry quite thin. Leave an overhang and then put in the freezer while you roll out the other 1/3 for a lid.

Step 5

Tip the onions and potatoes into a bowl and add the cheeses and cream. Taste and add a little more seasoning if it needs it. Gently stir together, then tip into the pie dish. Put the pastry lid on top and trim the edges so the bottom and top are the same size, then crimp with your forefinger and thumbs, to seal. Wash with egg, then bake at 190C/fan 170C/gas 5 for 40 minutes until deep golden. If freezing, cool completely, then cover and transfer to the freezer. To reheat, defrost in the fridge overnight, then heat the oven to 180C/fan 160C/gas Unwrap the pie and reheat for 20-30 minutes until piping hot. Leave for 10 minutes before cutting into wedges.