First, make an Italian meringue. Beat the egg whites with 100 g of the sugar to firm peaks. Meanwhile, boil the remaining 200 g sugar with the water until it is bubbling and the water has evaporated. This is hard to judge, but you should be able to see that there has been a reduction in the level of the liquid. At this point the sugar should still be white and syrupy. If it has started to change colour, you have gone too far. If you use a sugar thermometer, you can’t go wrong – just work to 120°C.
Pour the sugar syrup in a steady stream onto the beaten egg whites, beating on a medium speed so that the mixture cools as it is beaten. Place the bowl in the fridge to cool completely.
Beat the cream to soft peaks and gently mix in the mascarpone. When the meringue is cold, fold it into the mascarpone cream.
Fold in the pistachios, which add a nice bit of colour. Pour the semifreddo into moulds of your choice and freeze.
This is a fairly rich dessert and only needs some fresh fruit to accompany it. In the photograph it is served with pistachio praline. Make the praline by stirring pistachios into caramel and pouring into a lightly oiled metal tray. When the caramel sets, crush finely and use as a sweet garnish for desserts.