To a make the pastry a food processor: Add the flour salt and butter. Pulse until the butter is broken into small balls. With the processor running, very slowly drizzle in the water until it pulls away from the sides of the bowl and starts to form a ball. Stop the water immediately. You may not use all the water, or you may need a little more.

To make the pastry by hand: Add the flour, salt and butter to a large bowl. Using your fingertips (or a pastry cutter) rub the butter and flour until it resembles breadcrumbs. Drizzle in the water and mix using a fork until the it starts to come together and holds together when pressed in your hand. You may not need all of the water, or you may need a little more.

Turn the dough out onto a floured surface. If the dough is too crumbly, add a little more water in tiny amounts. If too dry, do the same with flour. Roll the dough to a rectangle. Fold the edge furthest away from you to the center and fold the edge closest to you to the center so the edges meet. Turn ¼ and roll out to a rectangle and fold the same way again. This step will make the dough light and flaky. Shape into a flat round, wrap in plastic wrap and refrigerate for 1 hour.

Add the potato to a large pan and cover with water, Boil for 12-15 minutes until soft when pierced with a fork, drain and mash.

While the potato cooks, add the oil and butter to a frying pan over medium heat. When the butter bubbles, add the onion and cook, stirring until softened. Remove and transfer to a mixing bowl along with the mashed potato and cheese, mix well. You want soft creamy mash, not crumbly. If it is too dry, add a little warm milk. Taste and add salt if needed.

Preheat oven to 400°F/200°C. Cover a baking sheet with parchment paper.

Brush all pasties with the egg/milk wash. Cut 2 slits in the tops of each.

Bake for 15-20 minutes, until golden brown. Time will vary depending on your oven.