1/2 lemon - juiced and made up to 125 grams with cold water
Mix the flour and with salt in a bowl
Cut up the cold butter into the flour
Add the lemon juice and water and mix by cutting repeatedly through the mixture with a table knife until it comes together into a shaggy ball with small lumps of butter dotted through it.
Tip the pastry onto a well floured worksurface and roll out into a rough rectangle about 1 cm thick. Fold the top third into the middle and then lift the bottom third over it. Seal the seams by pressing down the edges with the rolling pin
Turn the pastry block 90 degrees clockwise and repeat the rolling and folding procedure once more. Rest in the fridge for 10-15 minutes.
Repeat the rolling, folding and turning procedure twice more, this time rolling it out into a larger rectangle about 0.5 cm thick so that the lumps of butter begin to spread out and merge. Rest in the fridge for 10-15 minutes.
Repeat the rolling, folding and turning procedure twice more, this time trying to keep the rectangle as straight and even as possible.
The finished pastry block should look even, smooth and without lumps of butter breaking the surface. If it is not, then repeat the folding and resting process until it is.
Cover the pastry with clingfilm and rest in the fridge for at least an hour before using.