Preheat the oven to 200 ℃ / 180 ℃ fan / 400 ℉ / gas mark 6, then line two baking trays and take one roll of pastry out of the fridge.
Unroll the puff pastry and cut it in half lengthways, then into 8 rectangles. Move 4 of the rectangles to a lined baking tray, leaving space between them.
Add a fat pillow of filling to the rectangles, leaving a border free of filling around the edge.
Rub a little water around the edge of the filled pastry rectangles, then add another piece of pastry on top as a lid. I give each top piece a gentle stretch to make it a little wider and longer than the bottom piece to allow it to stretch over the filling.
Press down along the edges to seal the pastry, then use a fork to press down all around the edge to make sure it's a good seal and leave a nice finish.
Do this for all four pastries, then cut slits in the top of the pastry to let out steam (no soggy bottoms here!). I just add 3 or 4 slits, but Greggs cut slits in v shapes (in a chevron pattern). It's up to you, go as plain or fancy as you like. It's only there to let out steam as they bake.
Brush the pies with oil for that golden finish. While the first batch of four is in the oven, make the second batch the same way or wait and put them both in at once.
Bake them side by side or on different shelves, then swap them about part way through the bake. They will take 20-25 minutes.
Once baked and golden, serve and enjoy!