Put a large shallow-ish earthenware dish or a big loaf pan lined with cling wrap into the freezer to chill.
To make the semifreddo you’ll need 3 large bowls.
Whisk the sugar and egg yolks in the largest bowl until pale, creamy and almost doubled in size.
This will take a few minutes but don’t be tempted to cheat, this is what will make your semifreddo lovely and light.
In a second bowl, whip up the cream until you get soft peaks forming on the end of your whisk.
Don’t over-whip though – it’s better loose than thick.
Whisk the egg whites with a pinch of salt in your third bowl until you get very firm stiff peaks.
Add the cream and egg whites to the sugar and egg yolks.
Add the graham crackers, chocolate and orange zest, then gently fold into the creamy mixture.
Mix in most of the nuts, reserving a handful for serving.
Working quickly, scoop your semifreddo into your pre-frozen earthenware dish, or a loaf pan lined with plastic wrap and return it to the freezer as soon as you can.
It will take at least 4 hours to freeze until firm.
Take the semifreddo out of the freezer and pop it into the fridge for about 20 minutes before you serve it, so it goes from being hard to being lovely and scoop-able.