Banyan Branches

1 pound russet potatoes 1 large egg 1 large egg yolk 6 ounces Parmesan, finely grated, divided 1 cup all-purpose flour plus more 1 teaspoon kosher salt plus more 1/2 teaspoon freshly grated nutmeg 1 cup heavy cream 1 sprig rosemary 1 1/4″-thick slice fresh ginger 2 tablespoons (1/4 stick) unsalted butter, room temperature, . . . → Read More: Gnocchi Gratin