For the potatoes: Boil the russets in salted water until fork tender, about 15 minutes. Drain and let cool. Pass the russets through a ricer into a big bowl or mash by hand.

For the sausage: In a frying pan, heat the olive oil over medium heat. Remove the sausage meat from the casings and crumble. Fry until crispy, about 10 minutes. Drain the sausage and put in a bowl.

For the potato mixture: In a large mixing bowl, add the mashed potatoes, eggs, mozzarella and Parmigiano cheeses. Throw in the crisped sausage, parsley, 1 teaspoon sea salt and lots of freshly cracked pepper. Mix until almost smooth.

Preheat the oven to 350 ° F.

Grease a baking dish with 1 tablespoon butter. Lay down half the potato mixture and a layer of smoked mozzarella. Top off with the remaining potato mixture, small pats of the remaining 2 tablespoons butter and sprinkle with the breadcrumbs. Bake until the crust is golden, about 30 minutes.