Peel the potatoes and boil them. Mash the potatoes and add 50g of butter, eggs, mortadella, Fontina and salt.
Grease a cake tin with the remaining butter. Sprinkle some breadcrumbs on the bottom of the tin and then tip in the potato mixture. Sprinkle more breadcrumbs on top and add little knobs of butter.
Bake in the oven at 180 C / Gas 4 for about 30 minutes.
If you don't have mortadella or Fontina cheese, you can use bologne or another cooked sausage for the mortadella, and Emmental, Gruyere or Provolone for the Fontina. You could also omit the mortadella for a vegetarian version.