To make the pastry rub the butter into the flour in a large mixing bowl until it resembles breadcrumbs
Add the eggs and mix it together until a dough forms. Wrap the ball of pastry in cling film and chill it in the fridge for 30 minutes
Pre-heat your oven to 180C Fan/200C/400F/Gas Mark 6
To make the filling, fry the chopped onion in the vegetable oil for 5 minutes in a pan
Add the potato, Marmite and cheese, and stir everything together until it's all coated with the Marmite, take the mixture off the heat and leave it to cool
Roll out the pastry onto a mat or work surface dusted with flour, for large pasties use an 8" side plate or cookie cutter to cut out rounds
Spoon a generous amount of filling onto half of the circle, brush the edges with the beaten egg, then fold it over and crimp the edges with a fork
Place the pasty onto a lined baking tray and brush more beaten egg all over the whole pasty, then cut two small slits in the middle with a sharp knife
Bake them for 25 minutes or until the pastry is golden all over, you can lift them up gently to check the pastry underneath is cooked through
Serve hot, or allow to cool and re-heat