Roll out your flaky pastry on a lightly floured surface to about ¼ inch thick and cut out circles measuring 5-6 inches in diameter. Chill the circles in the refrigerator.

In a mixing bowl, combine grated cheddar cheese, sautéed onions, and mashed potatoes. Season with salt and pepper, mixing until evenly combined.

Place spoonfuls of the filling on each pastry circle, fold over to form a semi-circle, and crimp the edges securely.

Freeze the filled pasties for 30 minutes to maintain their shape during baking.

Preheat oven to 375°F (190°C). Line a baking tray with parchment and bake the pasties for 20-25 minutes until golden brown.

Let the pasties cool for a few minutes before serving warm.