Preheat the oven to 120°C (100°C fan-forced). Line a baking tray with paper.
Blitz the pistachios very briefly in a food processor, leaving some large fragments.
Using an electric mixer with a whisk attachment, whisk the egg whites together until soft peaks form.
Whisk in the sugar a little at a time, and continue to whisk for about 3 minutes at high speed until the mixture is thick and glossy.
Fold in the remaining ingredients, gently mixing until everything is incorporated.
Pour the mixture onto the tray, and smear it flat with a spatula if required.
Turn the oven down to 90°C (70°C fan-forced), and cook for 90 minutes. Turn the oven off and leave it there until the oven is no longer warm (about 2 hours).
Store in an airtight container until ready to serve. Either cut it into wedges or crumble it into smaller pieces.
Serve it with whipped cream and fresh berries.