1) Preheat the oven to 180C/Gas 4. Prepare a 28cm by 18cm by 4cm deep baking pan by lining with greaseproof paper.
2) Combine the sugar and cardamom in a small bowl, and mix with a spoon to combine. Beat the egg whites in a large bowl with an electric mixer on high speed until stiff, then beat in the sugar mixture (1 tbsp at a time) until the egg whites are thick and glossy. Add the flour, salt and pistachios, and mix with a spatula to combine.
3) Spread the mixture into the prepared pan. Bake in the oven for 30 minutes, until firm to the touch and golden on top. Cool for 10 minutes on a rack.
4) Meanwhile, reduce the oven temperature to 140C/Gas 1. With an electric knife, or a very sharp knife, cut thin slices (1/2cm wide, 18cm long) and place on a large cookie sheet. Bake for 20 to 25 minutes longer, or until crisp, dry and firm. Store in an airtight container.