1. Over a double boiler and making sure the bowl does not touch the simmering water, heat egg whites and sugar. Stir over the heat until the whites are hot to touch, or registers 50 – 60˚C. If you don’t have a thermometer, try putting your finger in; if it feels hot to touch, it’s done. Using an electric whisk, whisk the warm egg whites and sugar until soft peaks form. Scrape vanilla beans out of the pod using the tip of a knife. Add into the egg whites and whip for a few seconds. Separately, in a clean, dry, large bowl, combine whipped egg whites, cream, pistachio, and rose petals. Do not overwhip, as this could cause the mixture to collapse. Pour into prepared tin mould. Sprinkle coarsely-chopped pistachio and rose petals over the semifreddo as garnish. Cover in cling wrap and freeze for a minimum of 12 hours.