Wrap the butter in baking parchment or use its own wrapper and freeze for a few hours until solid, or leave overnight.

Tip the flour into a large bowl, then coarsely grate in the frozen butter, dipping the end in a little of the flour occasionally to prevent clumping. Pour in the buttermilk and swiftly combine using a cutlery knife. Turn out onto a work surface and use a bench scraper to bring it together into a dough. Cut away a third, then shape both portions into discs. Wrap and keep chilled while you make the filling.

Put the bacon in a cold frying pan and set over medium heat. Fry until crisp, golden and the fat is released, about 5-6 mins. Remove to a medium bowl using a slotted spoon.

Melt the butter in the pan over a medium-low heat and cook the onion for 8-10 mins until softened and golden. Turn the heat up slightly, add the garlic and cook for 2-3 mins more until fragrant. Transfer to the bowl with the bacon and leave to cool for 10 mins.

Meanwhile heat the oven to 220C/200C fan/gas 8 with a large baking tray inside. Roll the larger pastry disc out on a lightly floured surface until it’s roughly 30cm in diameter, or large enough to line a 23cm pie dish. Lift it into the dish and smooth out gently. Roll the smaller disc out until it’s also about 30cm in diameter, and set aside.

Pour the beaten eggs into the bowl with the onion and bacon, whisking to combine, then stir in the mustard, crème fraîche, nutmeg and chives. Season with plenty of black pepper and a little salt. Pour the mixture into the pastry case. Dampen the edge of the pastry using a little water, then lift the other pastry disc over the top. Press a fork around or crimp the edge to seal. Cut a small cross into the centre to allow steam to escape, then brush the remaining beaten egg all over the top of the pie. Bake for 40-45 mins until golden.

Leave to cool completely before serving with cornichons and more mustard, if you like.