Line a 900g (2lb) loaf tin with a long strip of greaseproof paper that lines the base and goes up the shorter sides of the tin to leave overhang – this makes it a lot easier to remove – or you can fully line the tin with clingfilm. Place the tin in the freezer to chill.
Place half the berries in a small saucepan. Simmer on a low heat for around 20 minutes until the fruit has broken down, then strain through a sieve into a bowl and allow to cool.
Mix the lemon curd with the limoncello (if using) and place to one side.
Meanwhile, place the egg yolks and sugar in a large mixing bowl and whisk until pale, then stir in the lemon zest. In a separate bowl, whisk the cream to soft peaks. In another bowl, whisk the egg whites to stiff peaks. Carefully fold the cream and egg whites into the egg yolk mixture using a large metal spoon. Remove the tin from the freezer. Spoon half the semifreddo mixture into the lined tin. Ripple in half of the blitzed berry mixture and scatter in some whole berries, then spoon over the lemon curd mixture. Spoon over the remaining semifreddo mixture and ripple in the remaining berry mixture. Reserve the remaining whole berries for serving. Place the semifreddo in the freezer uncovered for 4 hours, then cover and freeze for at least another 4 hours or overnight.
When you’re nearly ready to serve, remove the semifreddo from the freezer and place in the fridge for 20 minutes. In a food processor, blitz the shortbread and pistachios together to make a crumb. Turn the semifreddo out onto a serving plate or board and scatter over the crumb. Slice and serve with the remaining fruit.