About 1½ cups shelled pistachios, plus more for sprinkling on top
¾ cup sugar
6 large egg whites
2 cups very cold heavy cream
Piece of dark chocolate for shaving on top (optional)
Line a standard loaf pan (8 ½ by 4) with plastic wrap, leaving about a 2-inch overhang on all sides. Set aside.
Place 1 cup of the pistachios in a food processor with ½ cup sugar and pulse until very finely ground. Toss in the remaining ½ cup pistachios and pulse to coarsely grind. (You want the texture of both finely and coarsely ground nuts for the semifreddo.)
Beat the egg whites in a chilled bowl with an electric mixer with chilled beaters. (This keeps everything smooth and creamy.) Beat just until stiff soft peaks form.
Whip the cream, adding the remaining ¼ cup sugar a little at a time. Continue whipping until still peaks form. Using a rubber spatula, gently fold in the pistachio nut mixture until incorporated.
Fold in the egg white mixture next, stirring gently but thoroughly, until incorporated.
Pour the mixture into the prepared loaf pan. Cover and freeze until firm, about 5 hours.
To serve, unmold onto a plate. Peel off the plastic wrap. Slice and serve sprinkled with additional chopped pistachios and perhaps a bit of shaved chocolate. Alternatively, scoop the semifreddo into cups or bowls and sprinkle with the pistachios and chocolate.