Begin by melting 1 tablespoon of butter in a large frying pan over medium heat. Add the diced potatoes and fry for about 15-20 minutes, stirring occasionally, until they are crispy and golden. Season with salt and pepper, then remove from heat and allow to cool slightly.

Once the potatoes are cool enough to handle, stir in the grated onions. Cook for an additional minute until the onions soften and release their sweetness. Remove from heat and set aside to cool completely.

In a large mixing bowl, combine the cooled potato and onion mixture with 150g sharp cheddar cheese, 1 teaspoon Dijon mustard, 1/4 cup heavy cream, and optional 1 teaspoon fresh parsley. Mix until well combined.

Roll out the chilled puff pastry until it's about 3mm thick. Cut into four equal rectangles. Place a generous spoonful of the filling on one half of each rectangle, brush edges with beaten egg to seal, fold over, and crimp edges with a fork.

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