Cover potatoes with salted cold water by 2 inches in a 5-quart pot.  Bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, 18 to 20 minutes.  Shortly before potatoes are done, bring milk to a simmer in a small sauce pan over moderate heat, then remove from heat. Put oven rack in middle position and preheat oven  to 400 F.  Butter a 2 – to – 2 1/2 quart shallow baking dish and sprinkle with bread crumbs.  Drain potatoes well in a colander.  When cool enough to handle, peel potatoes and return to pot along with hot milk, salt, pepper, nutmeg and 3 tablespoons butter.  Mash with a potato masher until combined well and almost smooth.  Stir in egg, mozzarella, salami (or prosciutto) and 1 cup Parmigiano-Reggiano.  Spread potato mixture evenly in baking dish.  Sprinkle with remaining 2 tablespoons Parmigiano-Reggiano and dot with remaining tablespoon of butter. Bake until top is browned, 35 – 45 mintues. Let potato pie stand 5 mintues before serving.