Line a swiss roll tin with a sheet of puff pastry, bunching up the sides. Lay the potato slices over the puff pastry, slightly overlapping each slice. Sprinkle with sea salt and pepper to taste. I don't put loads of salt, since there's the saltiness of the bacon and cheese.
Sprinkle the chopped bacon or pancetta (you could use cubetti di pancetta) over the potatoes. Then cover with the grated cheese and chopped rosemary. Finally, brush the sides with the beaten egg.
Bake in the preheated oven for 30 to 45 minutes, until cheese is browned and potatoes are tender.