Rotolo di melanzane

Ingredients:
1 sheet of puff pastry (store-bought or homemade)
2 large potatoes, peeled and thinly sliced
150g prosciutto (or cooked ham), sliced
1 onion, finely chopped
2 tablespoons olive oil
Salt and pepper to taste
Fresh thyme or rosemary (optional)
1 egg (for egg wash)
100g grated cheese (Parmigiano, mozzarella, or a mix)
Directions: