Step 1
Place the potato in a large saucepan and cover with cold water. Bring to the boil over medium-high heat. Cook for 15 mins or until the potato is tender. Drain well and return to the pan. Use a potato masher or fork to mash until smooth. Set aside to cool completely.
Step 2
Meanwhile, add the bacon to a frying pan over medium heat. Cook for 3-4 mins or until the bacon is golden and crisp. Use a slotted spoon to transfer the bacon to a plate, reserving any fat in the pan. Add the leek to the pan and cook, stirring, for 5 mins or until the leek softens. Transfer to the plate with the bacon. Set aside to cool.
Step 3
Preheat oven to 200°C. Line 2 large baking trays with baking paper. Combine the potato, bacon mixture, parsley, rosemary, cheddar and parmesan in a large bowl. Season with pepper. Place 1 pastry half on a clean work surface. Divide the potato mixture into 10 even portions. Shape 1 portion into a 3cm x 24cm log. Place along 1 long edge of the pastry half. Brush edges of the pastry with a little egg. Roll up to enclose the filling. Cut into 4 pieces and place, seam-side down, on a lined tray. Repeat with the remaining pastry halves and potato mixture. Brush with the remaining egg.
Step 4
Bake, swapping trays halfway through cooking, for 25-30 mins or until the pastry is puffed and golden and the filling is heated through.