I used to buy pies like this in my local chip shop, as a lad. It’s real comfort food, warming and filling, based on potatoes but enhanced with lots of delicious cheese. The rich pastry is very tender and almost melts into the filling beneath. Serve this as a meat-free main course, with some steamed greens or a big bowl of salad.
To make the pastry, put the flour into a bowl. Add the butter and vegetable fat and rub them in lightly with your fingertips until the mixture looks like coarse breadcrumbs (this dough is richer and stickier than a standard pastry). Alternatively, rub in the fat using a food processor or a mixer, and then transfer to a bowl.
Add the cheeses to the flour and fat and mix well. Now, using a table knife, work in just enough cold water to bring the pastry together into a dough. You’ll probably need 2–3 tbsp. When the dough begins to stick together, use your hands to gently knead it into a ball. Wrap in cling film and place in the fridge to chill while you make the filling.
Heat your oven to 200°C/gas 6 and have ready a metal baking dish, about 24 x 20cm and 5cm deep.
For the filling, peel the potatoes and cut them into large chunks. Place in a saucepan, cover with water, add a little salt and bring to the boil. Reduce to a simmer and cook until the potatoes are tender enough to mash – probably 12–15 minutes.
Drain the potatoes well and put them through a ricer into a large bowl (this will give you smooth, lump-free mash but you can use a standard potato masher if you prefer). Add the onion, cheese, some salt and a good pinch of white pepper. Beat with a wooden spoon so the cheese is well incorporated. Add the milk and chives and give the mix a good stir.
Put the mash into the tin and use the back of a spoon spread it out until smooth and even.
On a lightly floured surface, roll out the pastry so it will just fit on top of the mash. It should be 8–10mm thick. Place it on top of the mash, trim the edges neatly and mark the pastry into 6 portions by scoring it with a knife.
Brush the pastry with beaten egg and bake for 20–25 minutes until golden. Let the pie stand for 15 minutes before cutting and serving.