Prepare the potatoes. If using raw potatoes, pierce them with a fork and microwave on high for 15–20 minutes, turning every 5 minutes, until tender. This method keeps them from getting watery and concentrates their flavour.
Set aside until cool enough to handle.
While the potatoes cool, melt the butter in a large saucepan over a medium-low heat.
Add the finely chopped onions and cook gently until soft and translucent, but not browned.
Remove the pan from the heat.
Peel the cooled potatoes—the skins should slip off easily.
Add the peeled potatoes (or your leftover cooked potatoes) to the saucepan with the onions.
Mash the potatoes directly in the pan.
Don't worry about a few lumps; they add a lovely texture to the filling.
Fold in the grated Cheddar and chopped sage, then season generously with sea salt and black pepper.
Mix well and resist the urge to eat the entire filling!
Preheat your oven to 170°C (340°F) / 150°C Fan / Gas Mark 3½. Lightly flour your work surface and roll out the puff pastry.
Cut out four 20cm / 8-inch circles.
Divide the cheesy potato filling evenly among the centers of the pastry circles.
Brush the egg wash around one half of each circle's edge.
Fold the pastry over the filling to create a semi-circle. Press the edges together firmly and crimp them along the top to seal.
Place the pasties on a baking sheet.
Brush the tops of the pasties with the remaining egg wash and sprinkle with a little sea salt for a crunchy finish.
Bake for 20–30 minutes, or until the pastry is puffed up and a deep golden brown.
Let the pasties cool on the baking sheet for at least 10 minutes before serving. The filling will be incredibly hot straight from the oven!
