Place butter, lard, flour & salt into mixing bowl
Gently rub the fat into the flour using finger tips until the texture resembles very coarse breadcrumbs.
Add 2-3 tablespoons ice cold water to bind the mixture
Lightly knead the dough until mixed, then dust with flour, cover and chill in the fridge for 30 minutes before using
FILLING
Heat the butter in a saucepan and fry the onions for 10 minutes, without colouring.
Stir in 150ml/5fl oz water, salt & black pepper and cook until almost all the liquid has evaporated. .
Remove the onions, spread out onto a plate to cool
Roll out two-thirds of the pastry until it is about 0.5cm/quarter inch thick and line a greased 20cm/8in loose-bottomed tart tin, leaving any excess pastry overhanging the edge.
Cover the base of the pie with half of the onions and then cover with half the grated cheese
Repeat this layering process until all the onion and cheese is used up.
Roll out the remaining pastry to make a lid for the pie.
Brush the edges of the pastry case with milk to seal the pastry lid upon it, while also pressing the edges together lightly before trimming off any excess overhang
Brush the surface of the pie with milk. Make three small incisions into the centre of the pie using the point of a sharp knife
Place the pie into the oven and bake for 40-50 minutes, or until golden-brown. Remove from the oven and leave for a good 20-30 minutes before removing