Preheat oven to 180 C/ 350 F and prepare a baking tray with parchment.
Using a fork or your hand, mix the sausage meat, chestnuts, shallots, cranberries, herbs and pepper (no salt at this stage) in a bowl.
Unfurl the pastry sheet (leave it on the paper in which it comes rolled up) and spread the sausage mixture all over it, as evenly as you can.
Snugly roll up both of the long sides until they meet in the middle. Use the paper to help you get started and don’t worry about the rolls being really tight as the pastry puffs up when it cooks.
Brush the pastry all over with the beaten egg.
Using a sharp knife, cut into approx 1.5 cm slices, laying each one out on the baking tray as you slice. You might have to re-form them a little on the tray. Leave some room between them as they do spread out on cooking. I managed to get 13 slices out of my roll, including the ends.
Egg wash the pastry again as well as the exposed filling. Don’t skimp on this step as it makes a huge difference to how beautifully burnished they look once they come out of the oven.
Sprinkle over a little flaky sea salt and more pepper.
Bake for 20 mins and let cool a little before serving.