There's something seriously comforting about a hot cheese and onion pastie - all that gooey cheese inside. Yum!
Make the Pastry
We're using a simple hot water pastry for this recipe. To begin, cut your butter into small chunks. Melt the butter into the water in a small pan on medium heat. Don't let the water boil.
In a large bowl, stir the salt into the flour and when the butter has all melted, stir half of the buttery water in. When that's all incorporated, stir in the rest. Finish mixing by kneading the dough by hand. Wrap in plastic film and place in the fridge to cool down to room remperature.
Make the Filling
Peel the potatoes and cut them into 1 centimetre (just under half an inch) cubes. Place them into a pan of boiling salted water and cook until tender - about 10 minutes. Peel and chop the onions and soften them in a little oil in a frying pan on medium heat. Don't brown the onion. Remove from pan and set aside. Grate most of your cheese, but keep some of it as whole cubes about 1cm on a side. We're going to make a standard cheese sauce, so melt the butter in the pan, stir in the flour to make a smooth paste, then add the milk and seasoning. Let it cook for 5 minutes or so, stirring all the time until it has thickened. Stir in the potatoes, and then the grated cheese. When the cheese has melted, take the sauce off the heat and let it cool down.
Assemble and Bake
Preheat your oven to 180°C (356°F) if it's a fan oven, 200°C (392°F) if it's not.
Roll out your pastry to about 4-5 mm (quarter of an inch) thick. Place a side plate upside down on the pastry and cut around its edge. Spoon a good couple of tablespoons of filling onto each disc, and put some of the whole cheese cubes on top. Moisten the edge of the disc with water, then fold it over to create a half-moon shape. Press the edges firmly together, then crimp the edge.
Make some holes in the top with a fork and brush with beaten egg mixed with milk. Sprinkle on some oregano for decoration. Place on a baking sheet and bake for 20 minutes. After this time, turn the baking sheet round and cook for maybe 10 minutes longer, until the pastry is golden brown.