1: Peel and thinly slice the potatoes, using a mandolin if you have one. Put the potato slices into a medium saucepan, cover with cold water and add ½ tsp salt. Bring to the boil over a high heat and cook for 2 minutes or until just tender (use a cocktail stick to check them). Drain the potatoes in a colander, then run under the cold water tap to cool them quickly. Set aside.

2: Melt the butter in a medium sauté pan over a medium-high heat then add the onion and sauté for 10 minutes or until softened. Add the garlic and thyme to the pan, along with some salt and pepper, and cook for another 10 minutes or until the onion is caramelised.

3: Tip the onion mixture into a bowl and let cool slightly then stir in the mustard and crème fraîche; set aside.

4: Line a baking tray with baking paper. Split the puff pastry into two-thirds and one-third portions. Roll out the smaller portion of pastry on a lightly floured surface to a 25cm square. Cut out four 12cm rounds, using a small plate or upturned bowl as a guide. Lay the pastry rounds on the lined baking tray and place in the fridge to firm up for 20 minutes or so.

5: In a small bowl, mix the grated Cheddar and Emmental together; set aside.

6: Have ready four 9cm diameter metal rings to assemble the pies (I use poaching egg rings). Take the tray from the fridge and gently place a metal ring in the middle of each pastry round.

7: Layer half the potato slices in the metal rings, top with a layer of caramelised onion and sprinkle on a quarter of the cheese. Layer the remaining potatoes on top, then the remaining onion and finally another quarter of the cheese. Place the tray back in the fridge to firm up.

8: Roll out the remaining pastry on a lightly floured surface to a 35cm square and cut into 4 equal squares. Take the tray from the fridge and carefully lift the metal ring off each pastry round.

9: Brush a little egg glaze around the rim of each pastry round then lay a square of pastry over the top of each pie, using your hands to secure the pastry around the filling. Trim away the excess pastry and press the edges together with a fork to seal.

10: Brush the pies all over with egg glaze and sprinkle the remaining grated cheese evenly on top of them. Place the tray of pies back in the fridge to .rm up for 20 minutes or until ready to cook.

11: Preheat the oven to 200°C/180°C Fan/Gas 6. Place the tray in the oven and bake the pies for 30 minutes or until the cheese is melted and the pastry is crisp and golden brown. Serve the pies warm from the oven, with mustard on the side.