Cook potatoes in boiling water in large saucepan over medium low heat until tender. Drain. Cool slightly; pare. Press potatoes through food mill or mash until smooth. Combine potatoes, melted butter, Parmesan cheese, egg, egg white, salt and nutmeg in large bowl until smooth; set aside. Sprinkle 1/2 of the bread crumbs in well-buttered 9-inch round baking dish. Tilt dish to coat. Spread about 1/2 of the potato mixture on bottom and sides of dish. Combine Fontina, Provolone and prosciutto in small bowl. Sprinkle over potato mixture in dish. Cover with remaining potato mixture; sprinkle with remaining bread crumbs. Dot with pieces of butter. Bake in preheated 350(F) oven 40 minutes or until thin crust forms. Let stand 5 minutes. Invert baking dish onto serving plate, tapping gently to remove. Serve immediately.