Peel the potatoes, cut them into coarse pieces and boil in salted water until they soften. Strain them and mash into a puree with a fork.
Bake the layer in a preheated 356°F (180 °C) oven for 20 min. until it hardens and turns a light pink. Take it out onto a new sheet of baking paper. Carefully peel off the old one in which you baked the layer.
Arrange round slices of ham on top of the layer, then the chopped mozzarella and wrap up into a roll, starting from the shorter side.
Put it in an oiled oven dish, smear lightly with oil on top and bake in a preheated 356°F (180 °C) oven for another 20-25 min. until reddened. Take it out onto a plate and cut it with a wavy knife. Serve with a garnish of your choice. Really easy, quick and tasty!