Chill a large bowl, a box grater and the butter in the freezer for 20 minutes before making the pastry.
Place the flour and salt in the chilled bowl and whisk together. Grate the butter using the chilled grater into the flour. Rub the butter into the flour until you have a pebble-like crumbly texture. You can do this by pinching and rubbing the butter and flour between your fingers.
Create a well in the centre of the mixture and add the sour cream and Cheddar cheese. Use a fork to slowly combine the flour by bringing it from the outside into the sour cream and cheese in the centre, until most of the flour is moist.
Lightly dust a work surface with flour and shape the dough into a disc. The dough should be firm and moist but not sticky. You want just enough moisture so the dough is barely sticking to itself. If it needs more moisture, slightly wet your hands when working with it or, if it’s too dry, lightly dust it with flour. Cover with plastic wrap and leave to rest in the fridge for 2 hours or ideally overnight
Using a rolling pin, roll the dough out into a 2 cm thick and long rectangle. Fold the dough into thirds as if you were folding a piece of paper to go into an envelope. Turn the dough clockwise and roll it back out into a 2 cm thick rectangle. Repeat this step six times until you have created many layers and the dough is smooth.
Divide the dough into two and wrap each piece in plastic wrap. Leave to rest in the fridge for a minimum of 15 minutes or up to 2 days. If you are not using within 2 days, you can store in the freezer for 3 months.
Apple galette
Preheat the oven to 200 degrees Celsius / 390 degrees Farenheit.
Take one disc of puff pastry. Sprinkle your kitchen bench or work surface with flour. Using a rolling pin, roll the dough out to a large circle that is roughly 4 mm thick and 25 cm wide. Roll the dough onto the rolling pin, then lay it over the prepared tray.
Core the apples. Cut them in half then slice the halves into very thin strips. Fan out the apple strips by pressing down onto them and pushing them away from you. Using a bench scraper or large knife, slide it under the apple slices and then transfer to the rolled out pastry sheet, starting from the outside edge leaving a 3cm boarder and working into the middle.
Sprinkle over the cinnamon and sugar. Place the small cubes of butter over the apples and tuck into any gaps.
Fold the outside 4 cm edge over, pinching the dough together to form a rustic circle shape. Brush the egg wash over all of the dough then sprinkle with the demerara sugar. This will help give the pastry a lovely golden colour. Bake for 20 minutes or until golden and crispy.
Leave to rest for 15 minutes before slicing into it. Serve with a big scoop of ice cream.