Line a medium sized glass bowl with cling film and allow extra to hang over the side of the bowl. Spoon in 1/2 of a tub of ice-cream and sprinkle over a few toasted pistachios. Place over a layer of sponge and soak with a little liqueur. Repeat this process 2 more times until you have filled the bowl. Cover the mixture with the excess cling film and freeze over night, or for at least 5 hours. 2 hours before you wish to serve the semi-freddo, make the ganache my heating together the cream and white chocolate. You may add in a little alcohol if you wish. Pour the mixture into a bowl and place into the fridge to cool and thicken. To serve the semi-freddo, remove from the freezer and submerge the bowl in hot water for just 10 seconds to release. Immediately unwrap the top layer of cling film and place your serving plate on top. Turn the whole this upside down and remove the bowl and the cling film. Working very fast, spoon the thickened white chocolate ganache over the top and decorate with fresh berries and toasted pistachios on top and around the edges of the semi-freddo. Dust with a little icing sugar and serve immediately.