Sieve the flour into a bowl. Add the grated butter.

Smoosh it all together with a fork, then get your fingers in and squeeze the butter through the flour until it goes like breadcrumbs. 

Make a well in the centre with a spoon and crack in the egg yolk. Start to mix that in from the centre out and pour in your lemon juice. Slowly add a little water and start working the mixture together with your hands. As soon as it's a 'rollable' consistency knead it a couple of times - not too much.

Wrap it in cling film and stick it in the fridge for at least 30 mins.

Now it's ready for your pie or tart or whatever! 

TIP: When you knead the pastry, before chilling, get it into a round shape, like a thick pancake. That makes it much easier to roll afterwards.