Gently melt the 85g butter in a saucepan and let it cool.
Sift the flour into the bowl of a stand mixer and add the water, butter and fleur de sel.
Beat at a low speed with a dough hook attachment until the dough is smooth and fairly firm, this will take around 3 minutes.
Flatten the dough into a square shape, cover in cling-film and leave to rest for at least 2 hours.
Take the remaining 335g of butter and place it between two sheets of greaseproof paper, roll the butter into a square around 13/15cm or a thickness of about 1cm. Wrap the butter in greaseproof paper and place it back in the fridge.
Roll the dough out into a square about 1cm thick, arrange the rolled out butter in the middle so that it looks like a diamond.
Fold the corners of the pastry into the centre of the butter like an envelope and make sure all of the butter is covered.
Run the rolling pin over the edges to smooth the surface, then roll the dough into a rectangle about 8-9mm thick, fold the top edge down two thirds, fold the bottom edge up to cover the top, so that it mimics a letter.
Cover the pastry in the cling-film and return to the fridge for 10 minutes.
Take the pastry out of the fridge and arrange it on a floured surface with an open edge facing you, roll the dough into a rectangle about 8-9mm thick.
Fold the dough into thirds like a letter and place it back in the fridge for 2 hours.
Repeat the last two steps twice more at 2 hour intervals, rolling out the dough and folding it like a letter.
To make the filling boil the potatoes until just soft, leave them to cool a little and then slice the potato thinly into thin squares.
Fry the onion and garlic in oil, add the thyme leaves and some salt and pepper, continue to cook until the onions have softened and they're tender.
Add the onions to the potatoes along with the double cream, mix thoroughly. Taste the mixture and add more salt and pepper if necessary.
Line two baking trays with baking parchment.
Divide the pastry into two pieces and roll out each on a lightly floured work surface until 2–3mm thick. Using an 8cm cookie cutter, cut out 16 rounds of pastry. Using a 9cm cookie cutter cut out another 16 rounds of pastry.
To assemble, spread the filling onto the smaller rounds of pastry leaving 2cm clear around the edge. Beat the egg yolk with 1 teaspoon of water and brush this eggwash around the edge of each pastry. Top each with a larger round of pastry, pressing the two pieces together to seal and place on the prepared trays. (Using a slightly larger piece of pastry for the top means that you can get a flush finish without stretching the pastry). Brush the pastries with the eggwash and put them in the fridge for 15 minutes. Preheat the oven to 200°C (180°C fan oven)/gas 6.
Remove the trays from the fridge and, using a knife, draw a decorative pattern onto the tops of the pastries. Bake for 30–35 minutes until the pastry is golden brown. Leave to cool on the trays for 5 minutes before transferring to a wire rack to cool completely before serving. These pastries keep well for a few days in the fridge in an airtight container.