Roll out the flaky pastry on a lightly floured surface until it reaches about ¼ inch thickness. Cut out circles around 5–6 inches in diameter. Keep chilled while preparing the filling.

In a mixing bowl, combine grated English cheddar, sautéed onion, and mashed potato. Season with salt and pepper to taste and mix until well combined.

Place a generous spoonful of the filling in the center of a pastry circle. Fold to create a semi-circle shape and crimp the edges tightly together, then set aside on a baking tray lined with parchment paper.

Chill the filled pasties in the freezer for about 30 minutes to help maintain their shape during baking.

Preheat the oven to 375°F (190°C). Bake the pasties for 20-25 minutes until golden brown and puffed up nicely.

Allow the pasties to cool on a wire rack for a few minutes before serving warm.