First, place the chopped potatoes in a medium saucepan, cover with cold salted water, and bring to the boil. Cook for 15-20 minutes, or until tender when pierced with a knife.

While the potatoes are cooking, heat the olive oil and butter in a large frying pan over a low-medium heat. Add the sliced onions and a good pinch of salt. Cook gently for 20-25 minutes, stirring occasionally, until they are very soft, golden brown, and sweet. Add the thyme leaves for the last 5 minutes of cooking. Learning how to properly caramelise onions is a brilliant kitchen skill.

Once the potatoes are cooked, drain them well and return them to the hot, dry pan for a minute to steam off any excess moisture. Mash them thoroughly until smooth, then set aside.

Preheat your oven to 200°C (180°C Fan) and line a large baking sheet with baking parchment.

In a large mixing bowl, combine the mashed potato, the caramelised onions, the grated cheddar cheese, single cream, and Dijon mustard. Season generously with salt and freshly ground black pepper. Stir until everything is well combined.

Unroll your sheet of puff pastry onto a lightly floured surface. Cut it into 6 equal rectangles. I find that using a pizza cutter gives a nice clean edge.

Spoon the cheesy onion and potato filling onto one half of each pastry rectangle, leaving a 1cm border around the edge. Be careful not to overfill them.

Brush the borders with a little of the beaten egg, then fold the empty half of the pastry over the filling. Press the edges together firmly and use a fork to crimp and seal them.

Carefully transfer the filled pastries to your prepared baking sheet. Brush the tops all over with the remaining beaten egg and use a sharp knife to make two or three small slits in the top of each pastry to allow steam to escape.

Bake for 20-25 minutes, or until the pastries are puffed up, deeply golden brown, and the filling is piping hot. What works best for me is letting them rest on the baking sheet for 5-10 minutes before serving. This allows the filling to set slightly and makes them easier to handle.