Tip the flour and a pinch of salt into a large bowl, or the largest bowl of a food processor. Rub in the butter using your fingertips, or, if using a food processor, pulse until the mixture resembles fine breadcrumbs. Gradually add 4 tbsp water, then continue to add more, 1 tbsp at a time, until the mixture just comes together into a dough. Tip the dough onto your work surface and gently bring it together with your hands, then shape into a disc. Wrap and chill for at least 30 mins before using.

Put the onions in a frying pan over a high heat, carefully pour in just enough water to cover, and bring to a rapid boil. Cook for 15-20 mins, adding a splash more water if needed, until the onions have softened and started to caramelise, and the water has evaporated. Tip onto a plate and leave to cool. Alternatively, put the onions in a heatproof bowl with a splash of water, cover and microwave on high for 8 mins, stirring halfway through. Leave to cool.

Heat the oven to 200C/180C fan/gas 6 with a baking sheet inside. Make up the sage & onion stuffing mix following pack instructions and set aside. Cut off a third of the chilled pastry and set aside in the fridge. Roll out the remaining two-thirds of pastry on a lightly floured surface into a large circle, the thickness of a 50p coin (ours was big enough to line a 26cm wide, 6cm deep pie dish). Use the pastry to line the pie dish and press it into the base and up the side. Trim the edges and use the offcuts to patch any tears or cracks, then return to the fridge.

Lightly beat 2 eggs, then stir this into the cooled onions along with the cheese. Season well with black pepper and a pinch of salt. Spoon the stuffing into the pastry case and gently press it into the base and up the side. Tip the cheese and onion mixture into the stuffing-lined pastry case. Beat the remaining egg and brush a little over the exposed pastry rim.

Roll out the remaining pastry as before to the thickness of a £1 coin – it should be slightly bigger than the pie dish. Lay this on top of the pie, sealing the edge with your fingertips. Trim any excess, then crimp the edge or press with a fork to seal. Poke a small hole in the middle to allow steam to escape, then brush the top with the remaining beaten egg. Put on the hot baking sheet and bake for 40-45 mins until golden brown all over. Leave to cool for 10 mins before slicing and serving, or leave to cool completely and serve cold. Will keep chilled for up to four days.