Crush or blend three-quarters of the pistachios to a fine powder. Roughly slice the remaining nuts. Preheat the oven to 200°C, gas mark 6. Cut the filo sheets to fit a baking tray about 12cm x 25cm. Brush the tray with a little melted butter. Layer half the filo sheets, one by one, in the tray, brushing butter on each (we use 8–10 sheets here). Do not press down; the sheets should lie loosely.