Sift the strong white flour and the pinch of salt into a large bowl. Mix the freshly squeezed lemon juice with 200ml (7fl oz) of the cold water, and pour it into the flour and salt mixture. Using your hands, mix to a firm dough, adding more water if necessary. This dough is called detrempe (a mixture of flour and water). Flatten it slightly and cover it with a plastic bag, cling film or greaseproof paper and allow it to rest in the fridge on a baking tray (which aids the chilling process) for 30 minutes.
Then, roll the detrempe into a rectangle about 1cm (½in) thick. Remove the butter from the fridge – it should be still in its wrapper – and, using a rolling pin, flatten it until it forms a slab about 1.5-2cm (5/8-¾in) thick. Remove the butter wrapper, place the butter in the centre of the dough rectangle and fold the dough over the edges of the flattened butter to make a neat parcel.
Dust the work surface with flour to prevent the dough from sticking. Turn the dough over on to the floured surface, and gently roll it out into a rectangle approximately 40cm (16in) long and 20cm (8in) wide, positioned so that one narrow end (the 20cm (8in) end) is facing you.
Brush off the excess flour with a pastry brush, then fold the dough neatly into three – lift the end furthest away from you and place it on the rectangle of dough, so that only one-third of the pastry is left uncovered. Align the sides as accurately as possible. Fold the other end on top. Seal the edges with your hands or a rolling pin.
Give the dough a one-quarter turn (90°) – the pastry should look like a closed book in front of you. Roll it out away from you, again into a rectangle (you should end up with roughly the same measurements as before), brush off any excess flour and fold in three again, as described above. Seal the edges, cover with cling film or greaseproof paper and allow to rest in the fridge for another 30 minutes.
Repeat the entire process twice more – every time you begin, start off so that the pastry looks like a closed book in front of you – so that, in the end, the dough has been rolled out six times and has rested in the fridge three times for 30 minutes each time.
Chill for at least 1 hour before using.