Start this recipe by making the semi-freddo at least 5 hours before you need it or the day before. Line the base and sides of a 20cm cake tin with baking paper (this will make removing the semi-freddo easy) Cut the maraschino cherries in half and place in a large bowl, reserving the juices. Add in the ricotta, nuts and the softened ice-cream and quickly mix together. Place a disc of sponge in the base of the prepared tin and soak with the cherry juices. Quickly spoon over half of the ice-cream filling and cover with the second piece of sponge. Soak with the remaining juices then spoon over the remaining filling and smooth out. Place in the freezer to firm up overnight of for at least 5 hours. 30 minutes before you are ready to serve the semi-freddo, make the ganache by melting together the chocolate and cream. Remove the semi-freddo from the freezer, peel off the baking paper and serve on a glass stand. Quickly pour over the chocolate ganache and sprinkle over the crushed pistachios. Serve immediately.