Carefully separate the sheets of pastry, cut to fit the baking tin and cover with a damp cloth.
Grind the walnuts and almonds finely in a food mill or food processor. Grind the pistachios also, but leave them a little coarser than the other nuts. Whisk the egg whites until stiff and mix in the honey, nuts, cinnamon and rose water.
Brush the baking tin with butter and fill with alternate layers of pastry and nut mixture, brushing each sheet of pastry with butter. Finish with a sheet of pastry.
Bake in a preheated oven (200°C/400°F with fan) for about 30 minutes.
Meanwhile put the sugar into a pan with 250 ml/1 cup water, bring to a boil and boil for 1 minute. Remove from the heat, let cool slightly and stir in the lemon juice.
Take the cooked baklava out of the oven and gradually soak with the syrup, while still hot. Let cool. Sprinkle with the ground pistachios. Cut into squares or rectangles to serve.