Roll out your flaky pastry to ¼ inch thick and cut circles of 5 to 6 inches in diameter. Chill in the refrigerator while preparing the filling.

Combine grated English cheddar, sautéed onions, and mashed potatoes in a mixing bowl. Season with salt and pepper, mixing thoroughly.

Place a spoonful of filling on each chilled pastry circle, fold to form a semi-circle, and crimp edges with a fork.

Chill the filled pasties on a lined baking sheet in the freezer for about 30 minutes.

Preheat oven to 375°F (190°C) and bake pasties for 20-25 minutes until golden brown.

Allow pasties to cool slightly on a wire rack before serving.