In a non-stick pan on a medium heat, melt the butter and sauté the leek and potato for about 10 minutes until softened. Add a small pinch of salt and a few turns of black pepper, and set aside to cool.
Unroll the puff pastry and cut it in half widthways. Place a small plate (about 15cm in diameter) on the pastry to use as a template, and cut around it. You should have enough room for two circles – one on each pastry half. Gather up the remaining pastry and, on a floured worktop, roll it out with a rolling pin before cutting out a third circle the same size as the first two. There will still be a little pastry left over, which you could cut into leaf shapes to add to the top of the pasties.
Place a tablespoon of the cooled potato-leek mixture on to one half of one pastry circle, and top with two slices of cheese.
Next, beat the egg and, using a pastry brush, brush the edge of the pastry circle with the egg before folding over to cover the filling and pressing down the edges well to seal. Repeat with the other two pasties.
Once you have completed all three, brush the pasties with the remaining egg and attach the leaf shapes, if using. Pierce a little hole in the top of each pasty to let the air out, then place on a baking tray lined with baking parchment and bake for 15 minutes at the top of the oven until golden brown.