You will need a square tray (20cm X 20cm) with at least 2cm high sides. Neatly line this tray with cling film and then brush the film lightly with vegetable oil. Set aside. In your food processor grind the pistachios to a powder and leave aside.

Now, place the gelatine and the orange blossom together in a heatproof bowl. Take a small saucepan, pour the orange juice inside and just bring to the boil. Pour the juice over the soaked gelatine whilst stirring until the gelatine has dissolved.

Have a sugar thermometer at hand. Place the sugar and water in a heavy based saucepan and slowly bring to the boil. Allow to simmer with the thermometer resting in the sugar until it reaches 125°C. Turn off the heat. Add the orange juice mix to the boiling sugar and stir to combine.

Now place the egg whites with the salt in the bowl of an electric mixer and start whisking on full speed. As the whites start turning pale and foamy, slowly drizzle the hot orange mix down the side of the bowl whilst the machine is running.

Leave whisking on high speed until the mix has cooled completely and resembles a thick meringue. It will take about 15 minutes.

Spoon the meringue into the lined tray with a rubber spatula then use a palette knife to level the mix evenly in the tray. Sprinkle the top with ground pistachios to cover and leave to set at room temperature about 2 hours.

Turn the marshmallow over onto another tray and peel off the cling film. Cover this side with more of the ground pistachios and using a very sharp knife, cut into squares.