Wash the potatoes and steam until tender.
Spear them with a carving fork and peel.
Push them through a potato ricer while still hot; repeat the ric ing operation, then push them through a fine sieve into a large mixing bowl.
This makes them easy to prepare.
Preheat the oven to 350°F.
Heat the milk together with 1/4 cup of the butter and a little salt and freshly ground pepper until scalding.
Stir most of the milk into the sieved potato; work in the eggs, Parmesan cheese, and the finely grated rind of the lemon.
The mixture should be fairly soft; add the rest of the milk if necessary.
Stir this mixture thoroughly for 2 minutes, then add the smoked cheese, the chopped Mortadella or ham, and the basil.
Grease a cake pan about 10 in in diameter and at least 2 in deep.
Place a quarter of the mixture in the pan and press out flat into an even layer with the palm of your hand.
Spread out slices of mozzarella on the surface and cover with another quarter of the mixture.
Repeat these last two layers.
Spread the sausage slices on top.
Sprinkle a covering of breadcrumbs over the surface and dot small pieces of the remaining butter on top.
Bake in the oven for about 30 minutes, browning under a very hot broiler for a final 5 minutes to form a crisp topping.
Remove from the oven and allow to stand for 10 minutes before serving.