Add all the ingredients to a food processor (except the water) and pulse to blend the fat into the flour – stop when the butter looks like oatmeal flakes.
Scrape everything into a large mixing bowl and drizzle over the water. Mix the dough together with a claw-shaped hand, then gather it together into a rough ball.
Tip the rough dough out onto a lightly floured surface. Smear heaped tablespoon-sized pieces across the work surface, spreading the dough about 15cm each time – this blends the fat into the flour, making for flaky, buttery layers.
When you have smeared all the dough, gather it all into a ball and knead briefly to make a homogenous dough. Wrap in clingfilm or baking paper and refrigerate for at least 2 hours, up to overnight.
When you are ready to roll out, bash the dough all over with a rolling pin to soften it, then roll the pastry about 2mm thick. Keep the surface lightly floured as you work, but don't use loads of flour, it will make the pastry dry and tough.
Using a 8cm round pastry cutter, stamp out as many rounds as you can. Gather the scraps together and roll out again, stamping out more rounds. Discard any leftovers, the dough will be chewy and overworked at this stage. Chill the pastry rounds while you make the filling.