Preheat the oven to 180°C/160°C fan.

If using pancetta, heat the oil in a large frying pan over a medium heat. Add the pancetta and cook for 6–8 minutes until crisp. Remove from the pan with a slotted spoon and set aside.

To the same pan, add the butter, leeks and mushrooms. Season and cook for 10–12 minutes until softened, then add the garlic and thyme leaves. Cook for a couple of minutes, then add the mustard and flour and cook for a few minutes more, stirring all the while.

Pour in the white wine and let it bubble away for a few minutes before gradually adding 250ml (9fl oz) of the stock, stirring constantly. Add the butter beans, cream and reserved pancetta, if using. Season well with black pepper and cook for 10 minutes. If it starts to look a little dry, add a splash more stock. Once it’s ready, pour the pie filling into a large pie dish and allow to cool completely.

Whisk the egg in a small bowl and use some of it to brush the top edges of the pie dish. Take the pastry and lay it over the pie filling – you may need to roll out the pastry slightly first, depending on the shape of your pie dish – then cut away any loose edges and crimp with a fork. If you’re feeling fancy, you can use any excess pastry to decorate the top. Brush the pastry with the remaining egg and season with salt. Bake for 35–40 minutes until the pastry is puffed up and deeply golden.

Leave the pie to rest for a few minutes, then serve in the middle of the table for people to help themselves, along with buttered peas.