Heat oven to 140C/fan 120C/gas 1. Line a baking tray with baking parchment. Place the pistachio nuts into a shallow roasting tin and toast in the oven for about 7-10 mins until golden. Place in a food processor and blitz until coarsely ground.
Blend the cornflour and vinegar to a paste. In a large clean bowl, whisk the egg whites until they form soft peaks, then gradually add the sugar a tbsp at a time until it forms a stiff glossy mixture.
Fold most of the pistachios into the meringue, reserving about 2 tbsp. Finally stir through the cornflour mixture. Spoon the mixture onto the baking tray to make four large meringue nests, using the back of the spoon to hollow out the centres. Sprinkle with the reserved pistachios and bake for about 45 mins until firm and crisp on the outside. Turn off the oven, but leave the meringues in to dry out in the warmth until completely cold.
To serve, place a meringue on each plate and spoon a little crème fraîche into the middle. Sprinkle over the seasonal berries and serve.