This iteration of a 17th-century French pithivier (from the town of the same name) comprises dauphinoise encased in two discs of cheat’s rough puff. The creamy cooking liquid is reinvented as a rich roquefort sauce.
Make the filling. Slice the potatoes very thinly either by hand or using a mandoline. Heat the cream, milk and garlic in a large pan. Add the potatoes, bring to the boil, then reduce the heat and simmer gently for 10-15 minutes, until the potatoes are tender but not soft. Drain the potatoes in a sieve, reserving the milky liquid. Spread the potatoes out on a large tray to cool, then chill until needed.
Heat the oil and butter in a large frying pan. Add the onions and cook on a low heat until soft, translucent and lightly caramelised. Remove from the heat and leave to cool, then chill until needed.
Make the cheat's rough puff. While the filing is chilling, mix the flour and salt together in a large bowl. Rub in the chilled butter using your fingertips until the mixture resembles breadcrumbs. Gradually add enough of the ice-cold water to form a ball of dough.
Lightly flour the work surface and roll the dough out to a 45 x 15cm rectangle.
Scatter half of the frozen grated butter over the bottom two-thirds of the dough. Fold down the top third and fold up the bottom third as if folding a letter.
Turn the folded dough 90 degrees and roll it out again into a 45 x 15cm rectangle. Repeat the process, adding the remaining frozen butter and fold as before. Wrap the dough and leave it to chill in the freezer for 15 minutes, or fridge for 30 minutes.
Repeat the rolling and folding process one more time, this time without the butter. Wrap the dough and leave it to chill in the freezer for 15 minutes, or fridge for 30 minutes.
Assemble the pithivier. Lightly flour the work surface and roll out just under half (two-fifths) of the pastry until about 3mm thick, then cut out a 26cm-diameter disc. Place the disc on the lined baking sheet and cover it with a sheet of baking paper. Roll out the remaining pastry and cut a second disc, about 28cm in diameter. Set the larger disc aside.
Layer the cold potatoes and onions on top of the smaller pastry disc, seasoning them between each layer with salt and pepper and a sprinkling of nutmeg. Aim to create a slightly domed shape and leave a 2cm border around the edge. Brush the pastry border with beaten egg, then gently lay the larger pastry disc over the filling. Press the edges firmly together to seal, then brush the top with beaten egg.
Scallop the base by using your index finger to push down on the edge of the pastry and the finger and thumb of your other hand to pinch the pastry either side.
Holding a small knife blade at right angles to the side of the pastry, 'knock up' the edges by making small indentations around the base. Brush the top of the pastry with more beaten egg to glaze, then chill the pithivier in the freezer for 15 minutes, or fridge for 30 minutes.
Brush the top again with the beaten egg and make a small hole in the centre of the domed top. Using the tip of a knife, score curved lines over the top, running downwards from the hole in the middle to the base, and taking care not to cut all the way through the pastry. Brush the top with beaten egg, then chill the pithivier in the freezer for a further 15 minutes, or fridge for 30 minutes.
Meanwhile, heat the oven to 200°C/180°C fan/400°F/Gas 6.
Bake the pithivier for 35-40 minutes, until the pastry is golden and crisp.
Make the sauce. While the pithivier is baking, melt the butter in a small pan on a medium heat. Add the flour and stir with a wooden spoon to a paste. Stir the paste over a low heat for 2-3 minutes to cook out the flour flavour, then gradually add the reserved milky liquid (from Step 1), stirring continuously to a smooth sauce. Add the roquefort and tarragon and season to taste with pepper (it shouldn't need salt).
Remove the baked pithivier from the oven and leave it to cool slightly before serving with the sauce for pouring over.