For the semifreddo, whip the cream until stiff and chill in the refrigerator. Separate the eggs. Place the egg whites in a large bowl in the refrigerator. Place the egg yolks the bowl of a double boiler. Slice open the vanilla bean lengthwise and scrape out the seeds. Add the vanilla bean seeds and the powdered sugar to the egg yolks. Simmer the water in the double boiler over very low heat and whisk the egg yolk mixture until thick and frothy. Then, place the bowl over ice water and continue to whisk until the egg yolk mixture is chilled.
Fold the whipped cream into the egg yolk mixture. Whisk the egg whites with the lemon juice until stiff. Fold the egg whites into the egg yolk mixture. Place the custard into a shallow metal bowl, cover, and freeze for at least 6 hours.
Remove the semifreddo from the freezer about 10 minutes before serving. Spoon the semifreddo into dessert glasses. To garnish, whip the cream. Serve the semifreddo dolloped with whipped cream and sprinkle with pistachios and cinnamon.