Preheat your oven to 200°C/400°/Gas 6. Put the potato, onion, cheese, mustard, egg and salt and pepper into a bowl and mix well.
200 g Potato, 0.5 Onion, 100 g Cheddar cheese, 1 Egg, 1 tablespoon English mustard, 1 pinch Sea salt and black pepper
Cut the pastry into 8 equal sized squares/rectangles.
375 g Puff pastry
Put 4 of the pastry squares onto a lined baking tray. Divide the cheese and onion mixture between the 4 pastry squares.
Place the remaining pastry squares over the cheese and onion mixture and crimp the edges with a fork to seal. Brush wit egg wash and make a small incision in the top of the pastry.
Bake for 30-35 minutes.
Don't take the pastry out of the fridge too soon, as depending on how hot your kitchen is, the pastry might get too warm and it's difficult to work with. Take it out only about 10 minutes before you need to, so it isn't too cold either, as that will cause it to crack.
The key to making these is not over stuffing the pastry, as they won't seal very well.
You can add some fresh parsley to lighten it up a little bit.
Make sure to generously season the mixture with salt and pepper.
Don't forget to make the little incision at the top of the pastry before putting them in the oven.
Be careful when eating, as the filling will be piping hot!