In a medium non-stick pan, melt the butter over medium heat. Add the diced potato and fry for 15-20 minutes until crisp and golden brown on the outside while remaining soft inside. Stir in the grated onion and fry for an additional minute. Season with salt and pepper, then transfer to a bowl to cool slightly.

Once the potato and onion mixture has cooled, combine it in a large mixing bowl with the sharp cheddar cheese, Dijon mustard, fresh parsley, double cream, and additional salt and pepper. Stir thoroughly.

On a lightly floured surface, roll out the chilled puff pastry into 1/8-inch thick sheets. Cut into four rectangles. Spoon filling onto one half, brush edges with beaten egg, fold over and crimp edges.

Preheat oven to 200°C (390°F) and line a baking tray with greaseproof paper. Place pasties on tray, brush tops with remaining egg and bake for 15-20 minutes until golden brown.

Remove from oven and cool on a wire rack for a few minutes before serving warm.